Despite Dragon Heaven currently being rather exploded it seems and despite the unfortunate cheating that occurred, I hope that everyone's 2023 has been going at least okay-ish so far even if the past four years have been rough--especially because they have, really. Any errors in movepool (and accidental brokenness) are mind since I just used the Gen 6 OU and National Dex Ubers settings on PS to check movepools against with DH currently being down. Let's get this "-Er -Eon" party started, shall we?
Mega Espeon
New Ability: Wind Reader - This Pokémon is immune to Wind moves and powder moves.
Type:
Stats:
HP: 065 → 065 (+00)
Attack: 065 → 085 (+20)
Defense: 060 → 080 (+20)
Special Attack: 130 → 150 (+20)
Special Defense: 095 → 115 (+20)
Speed: 110 → 130 (+20)
(BST: 525 → 625)
New Moves: Heat Wave (actually new), Power Gem ("new"), Weather Ball ("new")
Description:
Mega Umbreon
New Ability: Poison Eater - When this Pokémon deals direct damage to a poisoned or Poison-type Pokémon, it regains 1/8 of the damage done; this Pokémon cannot be poisoned; gaining this Ability while poisoned cures it.
Type:
Stats:
HP: 095 → 095 (+00)
Attack: 065 → 095 (+30)
Defense: 110 → 150 (+40)
Special Attack: 060 → 095 (+35)
Special Defense: 130 → 130 (+00)
Speed: 065 → 060 (-05)
(BST: 525 → 625)
New Moves: Crunch ("new"), Rapid Spin (actually new), Weather Ball ("new")
Description:
Sylveon-Mega
New Ability: Dragon Tamer - This Pokémon's Normal-type attacks cause the Dragon type to be added to the target, effectively making it have two or three types. Has no effect if the target is already a Dragon type. If an additional type would be added to the target, it replaces the type added by this Ability and vice versa.
Type:
Stats:
HP: 095 → 095 (+00)
Attack: 065 → 095 (+30)
Defense: 065 → 060 (-05)
Special Attack: 110 → 130 (+20)
Special Defense: 130 → 150 (+20)
Speed: 060 → 095 (+35)
(BST: 525 → 625)
New Moves: Mystical Fire ("new"), Play Rough ("new"), Weather Ball ("new")
Description:
It's a bit funny how I'm far more comfortable with my Sylveon-Mega design than Umbreon-Mega one despite liking Umbreon more--sorry, scorbunnys--given their shared stat distribution. It's probably because Sylveon-Mega can clearly actually get OHKO'd even before my slight, intentional sabotage of its Defense so that regular Sylveon could still have a slight advantage over it beyond Fairy-type screaming. I would lower Umbreon-Mega's defenses too if I could do so in a significant way, but alas. (Additionally funny how ability-wise Umbreon-Mega incidentally hates on Ghosts something fierce, especially Gengar, while Sylveon-Mega incidentally gets thwarted by them, though it and Drifblim-Mega just stare at each other awkwardly as far as abilities go.)
Mega EspeonNew Ability: Wind Reader - This Pokémon is immune to Wind moves and powder moves.
Type:
Stats:
HP: 065 → 065 (+00)
Attack: 065 → 085 (+20)
Defense: 060 → 080 (+20)
Special Attack: 130 → 150 (+20)
Special Defense: 095 → 115 (+20)
Speed: 110 → 130 (+20)
(BST: 525 → 625)
New Moves: Heat Wave (actually new), Power Gem ("new"), Weather Ball ("new")
Description:
Apparently a windy hill is one that I'm going to die on, though at least here it makes far more sense than it did for Mienshao-Mega given most of Espeon's Pokédex entries go on and on about how it reads air currents. Additionally, Cloud Nine was already taken two slates ago by Heliolisk-Mega--belated congrats to Gekokeso and all other legitimate winners--while that same entry pointed out why giving this Solar Power, which would definitely fit its flavor, probably wouldn't work out well even before it increasing Bug weaknesses on Sun teams and the weaknesses to First Impression and Sucker Punch undermining its speed and power given its fragility. So instead take this side-grade to Overcoat that's likely overall better since Wind moves cover more than Hail damage (R.I.P.) and Sandstorm damage do, even as tempting as it was to add the latter over powder moves despite that making far less sense. The temptation existed just because none of the Psychic mons currently immune to Sandstorm really care about that weather; admittedly, this Espeon-Mega wouldn'tt really care about Sandstorm either, but it would at least be able to use a really strong Rock-typed Weather Ball without being worn down--this even though I'm giving it Power Gem back too.
As it stands, the current Wind Reader would be quite good in both Singles and Doubles, especially with Amoonguss unfortunately still around even in this Kalos dex apparently and with the newfound immunity granted to various Doubles "wide" moves like Blizzard, Heat Wave, and Petal Blizzard. This version of Wind Reader also isn't broken like how I realized the initial version of this would be, especially in Doubles, when it was just Brambleghast's Wind Rider but with the +1 going to SpA (like it should in Brambleghast's case in a better world where that uses its SpA--sigh), so that's good, especially for Doubles. This mon doesn't exactly break the newest ground, especially given the middling stat spread boost that's intentionally modeled on Flareon-Mega rather than on my two other ideas for this slate, but that's not necessarily a bad thing for a mon that already has two really good abilities as a non-Mega and that could use a boost in its defenses both in terms of actual bulk and what it can come in on more than in anything else without being too obnoxious about either of them. (Besides, we all hate Amoonguss anyway, even more than Ferrothorn.)
As it stands, the current Wind Reader would be quite good in both Singles and Doubles, especially with Amoonguss unfortunately still around even in this Kalos dex apparently and with the newfound immunity granted to various Doubles "wide" moves like Blizzard, Heat Wave, and Petal Blizzard. This version of Wind Reader also isn't broken like how I realized the initial version of this would be, especially in Doubles, when it was just Brambleghast's Wind Rider but with the +1 going to SpA (like it should in Brambleghast's case in a better world where that uses its SpA--sigh), so that's good, especially for Doubles. This mon doesn't exactly break the newest ground, especially given the middling stat spread boost that's intentionally modeled on Flareon-Mega rather than on my two other ideas for this slate, but that's not necessarily a bad thing for a mon that already has two really good abilities as a non-Mega and that could use a boost in its defenses both in terms of actual bulk and what it can come in on more than in anything else without being too obnoxious about either of them. (Besides, we all hate Amoonguss anyway, even more than Ferrothorn.)
Mega Umbreon New Ability: Poison Eater - When this Pokémon deals direct damage to a poisoned or Poison-type Pokémon, it regains 1/8 of the damage done; this Pokémon cannot be poisoned; gaining this Ability while poisoned cures it.
Type:
Stats:
HP: 095 → 095 (+00)
Attack: 065 → 095 (+30)
Defense: 110 → 150 (+40)
Special Attack: 060 → 095 (+35)
Special Defense: 130 → 130 (+00)
Speed: 065 → 060 (-05)
(BST: 525 → 625)
New Moves: Crunch ("new"), Rapid Spin (actually new), Weather Ball ("new")
Description:
Ugh, trying to thread the needle on this one given all the more obvious options were taken and given that I wanted to keep all of these Megas as their singular type (though only Umbreon was ever really considered for getting an additional one) was difficult, and I still may have screwed up given the bulk of this thing combined with the options it already has. I simply wanted to go with a more active "I care about poison" theme without just doing something like Corrosion, though perhaps actively hating on Poison-type mons is a bit too much; I could see that needing to go just like how this originally just gaining back a flat 1/8 Max HP rate definitely needed to go since I eventually realized one could then just dump everything into bulk and become pseudo-immortal that way even before (STAB) Foul Play. The same goes for initially intending to give this Toxic Spikes before also realizing that made it a bit too self-sufficient, though it got Rapid Spin as both a notable compromise and to break the monopoly that, of all things, 
Morpeko still has on being an official Dark type with Rapid Spin--my kingdom for more Rapid Spin users in general.
Still, I think this is mostly fair enough as it is, with SpD specifically ending up the more "vulnerable" of its defenses simply because trying to make it more vulnerable on the Defense side would just have meant it got (a lot) more mileage out of both Foul Play and Curse sets, the latter of which probably becomes viable again anyway in the majority of the Umbreon-Mega forms already submitted. Additionally all Guts users want to use Flame Orb now anyway--except, ha, poor Flareon--and without Toxic Spikes as a new move for Umbreon, this is generally forced to forego other status moves too as well as still rely on its teammates more to spread Poison status if it doesn't want use spend a moveslot on a still inaccurate Toxic. It getting Rapid Spin is also incentive to use Toxic Spikes for it since they'll stay up when Rapid Spin is using and since its STAB Dark threatens Ghost mons anyway. Otherwise if not supported its ability just ends up as a glorified Immunity, which is one of the least powerful abilities that isn't nothingy or outright detrimental in the game, while at least Burn affects it more than it otherwise would now between the lack of Leftovers and Synchronize. This even especially with no Calm Mind in this format, which I intentionally didn't give it back, or the fact that the ability gets walled by Substitute unless you're using the now-unboostable and still weak Snarl. (Oh gods, I've just created a mon that probably fits best on Stall teams. What have I become?!)
Morpeko still has on being an official Dark type with Rapid Spin--my kingdom for more Rapid Spin users in general.Still, I think this is mostly fair enough as it is, with SpD specifically ending up the more "vulnerable" of its defenses simply because trying to make it more vulnerable on the Defense side would just have meant it got (a lot) more mileage out of both Foul Play and Curse sets, the latter of which probably becomes viable again anyway in the majority of the Umbreon-Mega forms already submitted. Additionally all Guts users want to use Flame Orb now anyway--except, ha, poor Flareon--and without Toxic Spikes as a new move for Umbreon, this is generally forced to forego other status moves too as well as still rely on its teammates more to spread Poison status if it doesn't want use spend a moveslot on a still inaccurate Toxic. It getting Rapid Spin is also incentive to use Toxic Spikes for it since they'll stay up when Rapid Spin is using and since its STAB Dark threatens Ghost mons anyway. Otherwise if not supported its ability just ends up as a glorified Immunity, which is one of the least powerful abilities that isn't nothingy or outright detrimental in the game, while at least Burn affects it more than it otherwise would now between the lack of Leftovers and Synchronize. This even especially with no Calm Mind in this format, which I intentionally didn't give it back, or the fact that the ability gets walled by Substitute unless you're using the now-unboostable and still weak Snarl. (Oh gods, I've just created a mon that probably fits best on Stall teams. What have I become?!)
Sylveon-Mega New Ability: Dragon Tamer - This Pokémon's Normal-type attacks cause the Dragon type to be added to the target, effectively making it have two or three types. Has no effect if the target is already a Dragon type. If an additional type would be added to the target, it replaces the type added by this Ability and vice versa.
Type:
Stats:
HP: 095 → 095 (+00)
Attack: 065 → 095 (+30)
Defense: 065 → 060 (-05)
Special Attack: 110 → 130 (+20)
Special Defense: 130 → 150 (+20)
Speed: 060 → 095 (+35)
(BST: 525 → 625)
New Moves: Mystical Fire ("new"), Play Rough ("new"), Weather Ball ("new")
Description:
And now we come to the last but certainly not least of these mons despite this mon having the ability that stymied me for the longest time of them all given everything else that's been taken so far. The ability might seem weird despite just being a rip-off of the autumnal Sawbucks-Mega's Autumn Leaves ability, but I've always personally found it quite funny two of Sylveon's Pokédex entries emphasized stories about Sylveon taking on dragons like they're acts of tremendous courage rather than ones of overwhelming type advantage. The ability also leans into the entries about Sylveon tending to touching enemies to condescendingly tell them to "calm down" before jumping them anyway.
I was actually going to lean into that little farce more by going a lot more physical and just giving this Intimidate or even Oblivious, but I wanted to be more creative (and masochistic) about things, and it seems fitting for arguably the (not broken) Gen 6 non-Mega mon with the new (broken) typing introduced in the same Gen and region to get an ability that focused around making Dragon-type into the liability it's (thankfully) become for its Mega form. Despite it being part of the reason I got stuck for a while, I thankfully realized that having the Fairy-moves themselves do this would be a tad overpowered, which neither Moonblast nor Fairy-type as a whole needs any help with. Hence the switch to Normal-type moves, even though that's also generally all the real physical coverage Sylveon gets anyway and even though it means Baby-Doll Eyes remains basically useless despite my initial goals--oh well.
Getting back to the Dragon Tamer ability, it can do markedly different things depending on whether its Singles or Doubles. In Singles, it basically only exists to help Sylveon (and team members) wallbreak through non-Ghost pokemon as well as help Sylveon bluff that it isn't a Mega by allowing it to still use Pixelate Hyper Voice before Mega Evolving to use once again Normal-type Hyper Voice to start the "you now dragon" train rolling with Moonblast as its real Fairy move. It can still backfire in Singles, especially against Steel mons where Dragon defensively covers a lot of the few vulnerabilities--Fire, Water, and Electric--they have left, but I figured that was fine compensation for how easily things can still get out of hand otherwise despite now being blanked by Ghosts and for this getting Mystical Fire back (from the future). Finally, in the case of Weather teams, it allows Weather Ball to actually remain somewhat threatening when the weather dies down instead of just becoming a wasted moveslot.
In Doubles, however, while the same offensive application applies, especially with spread Hyper Voice, you can also use it on your teammate to get rid of certain weaknesses or, in daring instances of 4D Chess, intentionally add them for purposes of things of like Weakness Policy with weak moves like Quick Attack while predicting (weak) Dazzling Gleams or Icy Winds. The Dragon type defensively is still mainly a liability in Doubles, but the fact that it does have some interesting resistances to add against at least a lot of wide moves helps it be less one-note than than the ability arguably is in Singles.
I was actually going to lean into that little farce more by going a lot more physical and just giving this Intimidate or even Oblivious, but I wanted to be more creative (and masochistic) about things, and it seems fitting for arguably the (not broken) Gen 6 non-Mega mon with the new (broken) typing introduced in the same Gen and region to get an ability that focused around making Dragon-type into the liability it's (thankfully) become for its Mega form. Despite it being part of the reason I got stuck for a while, I thankfully realized that having the Fairy-moves themselves do this would be a tad overpowered, which neither Moonblast nor Fairy-type as a whole needs any help with. Hence the switch to Normal-type moves, even though that's also generally all the real physical coverage Sylveon gets anyway and even though it means Baby-Doll Eyes remains basically useless despite my initial goals--oh well.
Getting back to the Dragon Tamer ability, it can do markedly different things depending on whether its Singles or Doubles. In Singles, it basically only exists to help Sylveon (and team members) wallbreak through non-Ghost pokemon as well as help Sylveon bluff that it isn't a Mega by allowing it to still use Pixelate Hyper Voice before Mega Evolving to use once again Normal-type Hyper Voice to start the "you now dragon" train rolling with Moonblast as its real Fairy move. It can still backfire in Singles, especially against Steel mons where Dragon defensively covers a lot of the few vulnerabilities--Fire, Water, and Electric--they have left, but I figured that was fine compensation for how easily things can still get out of hand otherwise despite now being blanked by Ghosts and for this getting Mystical Fire back (from the future). Finally, in the case of Weather teams, it allows Weather Ball to actually remain somewhat threatening when the weather dies down instead of just becoming a wasted moveslot.
In Doubles, however, while the same offensive application applies, especially with spread Hyper Voice, you can also use it on your teammate to get rid of certain weaknesses or, in daring instances of 4D Chess, intentionally add them for purposes of things of like Weakness Policy with weak moves like Quick Attack while predicting (weak) Dazzling Gleams or Icy Winds. The Dragon type defensively is still mainly a liability in Doubles, but the fact that it does have some interesting resistances to add against at least a lot of wide moves helps it be less one-note than than the ability arguably is in Singles.
It's a bit funny how I'm far more comfortable with my Sylveon-Mega design than Umbreon-Mega one despite liking Umbreon more--sorry, scorbunnys--given their shared stat distribution. It's probably because Sylveon-Mega can clearly actually get OHKO'd even before my slight, intentional sabotage of its Defense so that regular Sylveon could still have a slight advantage over it beyond Fairy-type screaming. I would lower Umbreon-Mega's defenses too if I could do so in a significant way, but alas. (Additionally funny how ability-wise Umbreon-Mega incidentally hates on Ghosts something fierce, especially Gengar, while Sylveon-Mega incidentally gets thwarted by them, though it and Drifblim-Mega just stare at each other awkwardly as far as abilities go.)
(be sure to profusely thank Exploziff for volunteering to help)
Espeon is the GOAT of Pokemon Colosseum. Not only is it fast and strong, but it also starts with Helping Hand to boost the power of its ally in a game entirely focused on double battles. As Espeon is filled with Mega Energy, the orb on its head glows, and it shares the excess energy with its ally in what is basically a constant passive Helping Hand. And now for someone who actually knows what they're doing in double battles to take the floor.
Alakazam in exchange for worse coverage and a lot less Speed. To fix this, Mega Espeon gets much better bulk and good coverage in the form of Focus Blast. You might think the Speed tier remains unchanged, but that's where the new Ability comes in.
Blunder Policy as a permanent Ability. This has a very interesting interaction with Zap Cannon, which becomes a coin toss: hit and Paralyze the opponent, or miss and get +2 Speed (either way, you will outspeed them). The aforementioned Focus Miss gains similar benefits, abusing the clearly-rigged RNG to your advantage. Even Sing can be used to create set up opportunities, becoming an Agility in the event that it misses.
Gengar ties it, and Alakazam and 
Mienshao, 
Pyroar, and 
Heliolisk all outspeed it. In NatDex, the Speed boost is even more valuable.
Mega Drifblim naturally defeats these by neutralizing Pixilate, so you may want to consider running Shadow Ball. All in all, Mega Sylveon can be pretty potent in the right hands.
