As the #1 Eeveelution fan here (Sylveon, Espeon, Glaceon, Vaporeon, Umbreon, Leafeon are all in my Top 15, most of them Top 10) I'm going to give my ideal Eeveelution buffs while being somewhat fair. I am going to make up new abilities because I feel like it's easier to do that for them rather than to use existing abilities and invent a theme in reverse.
This is not going to account for Tera (if you know, you know, but also I am expecting the next generation to have Tera). Also, all of the current Hidden Abilities are now regular secondary abilities. All of the abilities that exist are going to be outclassed.
I do not believe this mon needs a big buff, but my ideal new ability for it is
"Purification" based on the concept of Vaporeon being made of water, it self-purifying Poison status would be nice for it. As a unique ability, it's also immune to Water. This is a direct upgrade from Water Absorb, but considering this would be a Hidden Ability and Game Freak has in fact pretty much had Hidden Abilities be direct upgrades from regular abilities before, I don't mind. I think this ability alone would make it just a lot better in Singles with Toxic no longer being counterplay.
As for moves, I think it's in a very good place with a lot of utility, in both Singles and VGC. Even when I looked at what VGC support Pokemon are running, basically anything that makes sense thematically is something Vaporeon learns; Helping Hand, Yawn, Haze, Icy Wind, etc. The only move I could think of was maybe Life Dew, but I don't think that's actually used in any format. Dew just gives a water-y vibe, y'know?
Overall, this wouldn't make it much better in VGC, but VGC is much tougher to get Pokemon to a majorly viable state with. Tanky Water-Types simply aren't the type for the job without something like Gastrodon Storm Drain or Milotic with Contrary SWSH, Storm Drain maybe fitting but I feel it would still not compete with Gastrodon, nor do I think it'd be better in Singles with that, and Contrary not really fitting.
This Pokemon, on the other hand, definitely needs a big buff. It's tough because Electric is such a type that just relies on coverage with its immunity being one of the best types, and it being a part of it being one of the best types. The ideal ability I came up with, based on its theming is
"Ionic", and it's based on the Pokedex entries referring to Jolteon getting its power through negative ions. Every turn Jolteon remains in battle, up to a maximum of five, it gains a small (1.1x) power boost to all Electric attacks, and after the second boost it is able to hit Ground-Type Pokemon for neutral damage. After the attack, the charge resets. With its Speed, it'd be able to stall with Protect/Sub/else shenanigans after a switch in order to break the Ground immunity, I feel like this would make Jolteon not only better, it'd also make it IMO interesting as a competitive element. Obviously it can stall, but it has no recovery and using it in this way also means Specs is almost certainly not widely viable, and it isn't about to break down Blisseys. I think this would be balanced but make it interesting to work with, at least bumping it up a tier with the ability to gain all-but-Grass coverage for one turn every few turns. As with all three of these, this ability is also immune to Electric moves.
As for moves, I'm going to give Jolteon Dazzling Gleam and Grass Knot.
With a set where you mainly just run Thunderbolt with something like Substitute Calm Mind, you could play mindgames with if the Ground-Type will switch or not and try to go for midground plays if you still have the sub up. For VGC I think this + Electric Terrain would go crazy assuming that Gen 10 actually has a good setter. The main problem would be Amoonguss which still resists + has the ability to redirect, so I don't think it'd actually be that good, but I think it could still be a fun niche pick. For moves, I'd like to give it Charge Beam and Taunt.
Let's cut to the chase. This Pokemon is trash, as we all know, but I don't know what direction to take it on first glance. It feels like it should be something like Ursaluna, huge damage at the cost of longevity, but it has worse longevity because of the status it requires and it is too frail for the job. 65 HP and 60 Defense doesn't let it take those trades a Pokemon like Ursaluna can. We are going to give it a very good Hidden Ability.
"Insulation" is essentially a combination of several abilities, based on the fact that its fur coat is supposed to release heat into the air due to its body being like 1,600F. The first mixture of this ability concoction is Flame Body, Pokemon that make contact will be burned. The second part is essentially Guts with the ability for Flareon to get burnt still. The idea being that the Flame Orb or any other source of burn would cause it to start overheating, but that would let it do more damage. So it essentially has non-Fire-Type Guts, too. Lastly, it's immune to Fire moves as usual. I think this is a very convoluted ability, but if I'm honest I just couldn't find a way to just make Flareon work with just one function in its ability, and this Pokemon is still probably not even that good.
For moves we're going to give it Stomping Tantrum, Knock Off, U-Turn and if Thunderous Kick was no longer a Signature Move in Generation 10, give it that too. Fuck it. This shit's still like NU at best, but we try.
This Pokemon really isn't that bad, but with this I'd like to buff it to keep it relevant among power creep, but even more than that I want to differentiate it further from Vaporeon because the main difference is just that their monotypes have different resistances. Notably it's one of the few Eeveelutions to have never really gotten a good ability, so frankly I think giving it a busted ability is an over-buff. Even right now, with it almost having no ability for its role in Singles, it's been consistently good in the middle-of-the-pack tiers.
"Venomous Rings" takes, of course, inspiration from the fact this Pokemon was originally going to be a Poison-Type. It's not going to make it immune to Poison, since you can still poison creatures that secrete poison, and Umbreon is more spinned as a predator that happens to use poison rather than being naturally Poison-Type adjacent. However, it is going to have a 30% chance of the opponent being Poisoned when they make contact with Umbreon, punishing Physical Attackers for U-Turning it consistently and not breaking it. Taking some inspiration from Pokemon Unite, if a Pokemon is trapped by it or a teammate, Umbreon will deal double damage- this will only really matter with Foul Play if the target is a Physical Attacker, but may make it interesting in VGC or make for interesting Mean Look sets in Singles. As a fun bonus, something for the main game only, not any competitive format, Umbreon will also heal double HP when it is nighttime based on your system clock. This will not effect competitive.
As for moves, give it Follow Me for VGC specifically, and Knock Off more generally.
These buffs would make it better in this constant-pivot world, and I think it would be nice for it. Anywho,
Alright, I'm biased, this is my favorite Pokemon of all-time (and is literally my fucking
furry OC, so don't be surprised if I seem more explicit here) so I'm going to give it some stuff to make it way better. I am disappointed with how it went especially in early Scarlet/Violet formats, being one of the few fast Psychic-Types with actual power in the entire game was not enough, nor was its utility. I want to give it far more utility and make it stronger coverage wise. This one may be too big of a buff.
"Precognitive Fur" is based on the lore that Espeon's fur can immediately get the opponent's next move using the minutia of air currents generated. While it would be easy to take this logic in an accuracy direction, that's cringe so we're going to take the direction that it uses that prediction to reduce damage instead, partially dodging but not completely. This is a 0.67x modifier on Physical and Special attacks, status effects do the normal amount of damage. Now, allow me to defend this:
252 Atk Landorus-Therian U-turn vs. 0 HP / 0 Def Espeon: 250-296 (92.2 - 109.2%) -- 56.3% chance to OHKO
Now, let's assume that Espeon essentially has Intimidate:
-1 252 Atk Landorus-Therian U-turn vs. 0 HP / 0 Def Espeon: 168-198 (61.9 - 73%) -- guaranteed 2HKO
The non-STAB 80 BP super-effective attack only does an average of 65%! Yippee!!! Espeon gets to take one U-Turn! Of course, this will be more significant for Special Attacks, but it's not like Espeon has that much bulk there either.
252 SpA Enamorus Moonblast vs. 0 HP / 4 SpD Espeon: 166-196 (61.2 - 72.3%) -- guaranteed 2HKO
-1 252 SpA Enamorus Moonblast vs. 0 HP / 4 SpD Espeon: 111-132 (40.9 - 48.7%) -- guaranteed 3HKO
Now, don't get me wrong, I know this is a major buff that is probably cringe in of itself, but I think it would be cool so I'm writing it in anyways. This ability also has blockage of hazard stacking. It does not block other status moves, only hazard stacking. What is the explanation with the ability theme? Fuck you, that's what. This makes it have that utility of Magic Guard while not making theoretical Calm Mind sets impossible to check for tanky Special oriented Pokemon, such as Blissey. They can still status it and check it in other ways even if it has some boosts up, and that will allow physical threats to still deal with it- and they still will. Like Umbreon, as a cheeky easter egg, HP moves will heal double during the midday based on in-game clock. This does not effect competitive whatsoever.
As for moves, it's getting Mystical Fire. There's literal official merch of Espeon with a witch hat, I see it as a magician and I am the biggest Espeon fan, and I'm saying it's a witch so that's that. And now Game Freak has to give it Mystical Fire. As like other magician-type Pokemon like Delphox and Meowstic, we're giving it Nasty Plot too, fuck you.
Leafeon is a Pokemon that was doomed to fail because it's a Grass-Type, and that's really the theme of this duo in general. Leafeon got a dogshit offensive typing on an offensive build, and Glaceon got a dogshit defensive typing for an offensive build; quite a lot of dogshit going on in these two, despite the fact they're clearly foxes. Kanto Trio are dogs, Espeon and Umbreon are cats, Glaceon and Leafeon are foxes, Sylveon is a dog, debate over, the end. With the abilities I'm going to try to write some wrongs with the opposite-world type-to-build, and hopefully even out their properties. For Leafeon, moves will deal a lot of the heavy lifting with the ability just being there to help it branch out its builds if it wants to. The Pokedex says Leafeon in the wild do not fight, so a bulky build more similar to Tangrowth is something I'd enjoy seeing.
"Photosynthesis" is an ability derived from the fact Leafeon, well, uses photosynthesis in order to get its nutrients. When Leafeon is in neutral or Sunny weather, it heals Leftovers every turn for free. This makes it a much better tank than it was before, which it can pull off with a theoretical set running Knock Off and Leech Seed as utility moves. The heal doubles in Sunny weather, which will also combine with Leafeon having Chlorophyll included into the ability essentially. Leafeon has Base 130 Defense with a bad HP stat, so hopefully this can help it be a tank if it so pleases.
Giving it the moves Taunt, U-Turn, Triple Axel, Follow Me, Spiky Shield, Sleep Powder/Grass Whistle (if Grass Whistle still doesn't return), Stomping Tantrum and Wild Charge. That's a lot of moves, but I always felt the Sinnoh duo to have awful movesets.
I think this change wouldn't necessarily make Leafeon go up many tiers, but it'd make it have enough variety to endure in having some viability above ZU for a while, I'd hope. The moves let it be better as an offensive threat and give it more utility for a defensive set.
Garbage tier garbage, unfortunate for one of the best designs in the series. Awful Speed meaning it has to make use of its natural bulk in order to attack, which outside of HP isn't that bad, but the Ice-Type holds it back hard. We're going to give it a good ability for sure.
Glaceon's Pokedex entries mainly talk about how it can freeze the atmosphere around it to create diamond dust, so
"Diamond Dust" it is, and I'm going to take some inspiration from Pokemon Unite for this. For anyone who has never played Unite, Glaceon is bullshit in it, with it gaining these icicles with each auto attack which its skills use in order to lock on and target someone, using all of them at the same time from very far away. That is not my takeaway for the turn-based game of course, but I do think the idea of rewarding it for getting consecutive attacks is interesting. Every time Glaceon uses an Ice-Type attack, it will gain a 1.1x damage increase to its Ice attacks until switch out. This will be combined with the rest of the ability, which is weather themed, that will make you experience the Glaceon Unite wombo combo in turn-based form.
Firstly, we are combining Ice Body and Slush Rush into the ability. Secondly, if Glaceon hits a 1.5x on its own, it will summon Snow temporarily. This will make it a gigantic threat. This will only work once per switch in, so if you can weather the storm it will be neutered until next time, but I imagine it will be hard to deal with it at double speed with free Leftovers, the Snow +1 Defense boost and the fact it has Substitute Calm Mind. In fact, even my brain considers if this is too much synergy, but I think it'd be funny. Getting 1.5x on its own would also require it to get 5 Ice attacks in one switch in, so I also doubt that that will happen much if ever in a serious battle, so realistically you will need two Pokemon to get this to work. One may consider it privilege for Glaceon to get more buffs than regular Slush Rush ability Pokemon, and you're correct; Eeveelutions simply deserve privilege, next question.
As for moves, because I feel like I potentially made Glaceon extremely annoying as is, I'm not going to give it nearly as many moves as I did Leafeon. However, the other side of the dex entries notes it makes its fur act like spikes when it freezes them, so it gains Spike Shield with a bit of synergy with Leafeon. It also obtains Aura Sphere so that it can defeat Steel-Types, since I feel it is still never worth running if it can't deal with Steel-Types in any capacity. Ideally I'd prefer a Ground move, but Earth Power 90 BP feels a bit strong and for some reason the next lowest power Special Ground move is Mud Shot, which it learns so.
My second-favorite Pokemon of all-time! And also my furry OC! So yeah, I'm gonna buff it quite a bit. Not too hard, though, because this Pokemon is already sometimes viable. That being said I'd like it to be viable-er, and I feel like in Singles Pokemon like Clefable just outclass it too hard. Because Sylveon runs solo, I don't have any theme with another Pokemon to contend with. The original three I kept the immunity abilities, Espeon and Umbreon both had the healing thing and not much else similar I suppose (like the real games!) and Leafeon/Glaceon have their abilities largely tied to weather. Sylveon? I can do anything.
"Ribbons" is the final fanmade ability here, and it's based on the dex entries about it putting its ribbon feelers around its trainer's arm, in the original Y dex entry just because it likes walking with its trainer, in Ultra Moon's entry because it has a practical purpose of reading its trainer's emotions. For the sake of battle, what does this mean, however? It means that Sylveon will do better if it feels positive emotions and vibes; better Friendship with its trainer, the more damage it will deal. Less friendship, less damage. Essentially we will add a Return-like multiplier onto all attacking moves, maxxing out at 1.3x more damage for a free Life Orb. This will not combine with Pixilate because it does not get Pixilate back, it will have to settle for Dazzling Gleam to hit both opposing targets. On top of the effectively free extra damage, when it has max friendship it will also have Shed Skin effect applied at the end of each turn, kind of a reference to the affection features.
As for moves, Follow Me, Mystical Fire back from Generation 8 and Thunderbolt as another general offensive move, and that will round out its kit. It already has decent moves, and a lot of the moves I could add are moves that don't fit its character (like Taunt or Nasty Plot).
These changes will make Sylveon more of a general Attacker mage while giving it the ability to mostly-reliably get rid of status, letting it be a better tank with less disruption from other tanks or other moves. I think this could genuinely make it a really good Pokemon, and with its Speed being its lowest stats, I do not think this would break it. And if it does? It deserves it anyways.
I spent an hour and a half on this post already, so uhhh. That's all!