General ideas:
Gallade learns Focus Energy by level-up and Sacred Sword by Move Relearner.
Gallade's abilities changed: Steadfast -> Synchronize (from Synchronize Kirlia), gains Trace (from Trace Kirlia), Justified -> either Telepathy (able to read the minds of its allies to coordinate attacks, plus Gardevoir has this) or Forewarn (able to read the minds of its foes to plan its attack, as suggested by its dex entries)
Mega Gallade: Fairy/Fighting with Pixilate? Pixilate Slash new meta IMO. In terms of actual reasoning, Gallade is a Knight to defend the princess, just like in
fairy tales. Plus physical fairies are fun and Fairy/Fighting is nothing if not a unique typing.
Ralts line gets Frost Breath by TM (they learn Icy Wind so it makes sense to me, and in fact it doesn't make sense that they don't learn Frost Breath while still learning Icy Wind). In fact, you can probably freely give Frost Breath to any Pokémon that learns — or can be taught — Icy Wind while not having any logical way to perform the move other than breathing icy elemental breath.
Draining Kiss buffed from 50 base power to either 60 or 65 base power. (65 is a better choice in my opinion.)
Reasoning:
If Draining Kiss is 60 BP:
45 BP heal
105 total power (attack + heal)
If Draining Kiss is 65 BP:
48.75 BP heal
113.75 total power (attack + heal)
Giga Drain:
75 BP attack
37.5 BP heal
112.5 total power (attack + heal)
Meloetta retains her current Forme when switched out, as well as between battles. This allows her to be selected as Pirouette Forme in the teambuilder, and if she is switched out in Pirouette Forme, she'll remain in Pirouette Forme when switched back in. There's just one catch: In order to access the Pirouette Forme, Meloetta must still know Relic Song.
Meloetta is female, instead of genderless.
Meloetta learns Disarming Voice, Dizzy Punch, Double Kick, Sky Uppercut, Feint, Chip Away and Swords Dance by level-up. She also gains Snarl, Roar, Bulldoze, and Earthquake via TM.
Reasoning behind Disarming Voice: It's a voice move described as being a "sweet-sounding cry". Seems like a good fit in my opinion.
Reasoning behind Dizzy Punch: A punching move that suggests a precise, dance-like fighting style. A good fit for the Pirouette Forme.
Reasoning behind Double Kick: So she naturally gets a fighting move other than Close Combat. Also, Double Kick suggests an agile fighting style.
Reasoning behind Sky Uppercut: It's not Close Combat. Also, Sky Uppercut suggests an agile fighting style.
Reasoning behind Feint: A move that suggests an agile and flowing-yet-precise fighting style.
Reasoning behind Chip Away: Multiple rapid, weak hits add to a lot of damage. Suggests a fast, precise, and graceful fighting style.
Reasoning behind Swords Dance: It's a dancing move
and boosts Pirouette Forme's primary offensive stat! Why doesn't she get this by TM already?
Reasoning behind Snarl: It's a voice-based move, and while it may fall outside of Meloetta's natural tendencies, that's why it's a TM. Maybe she performs screamo to use this move? Or maybe it makes her a rapper? :P
Reasoning behind Roar: A voice-based move that is thematically fitting given what else I'm using to represent Meloetta, especially the Pirouette Forme. (And Aria Forme Meloetta can Roar with impunity, not having to worry about Magic Bouncers on the opposing team.)
Reasoning behind Bulldoze and Earthquake: This one might be a bit of a stretch, but it's sort of a mental image I found rather cool. Basically, Meloetta in her Pirouette Forme would be tap-dancing with precision and grace (like Riverdance if you've ever seen that)... but every tap of her foot against the floor causes a massive tremor to ripple through the ground. Fear the power of dance!
While you're at it, may as well consider other dancing moves than just Swords Dance. In particular, I was thinking Dragon Dance, Petal Dance, and possibly Fiery Dance, Lunar Dance and maybe even Quiver Dance. Lunar Dance and Quiver Dance seem like they're pushing it though.
Reasoning behind Dragon Dance: It's a dance move that boosts the Pirouette Forme's primary offensive stats. It's also learned by Pokémon that aren't Dragon-type anyway, so it stands to reason that Meloetta could learn it. Given that Meloetta's theme is song and dance, it doesn't make a whole lot of sense for her not to learn this move.
Reasoning behind Petal Dance and Fiery Dance: More of that whole "fear the power of dance" theme I've been going for with Pirouette Forme. Meloetta's dances can shake the earth, coat the world in flames, and summon the forests themselves against her foes!
Reasoning behind Quiver Dance: Another dancing move. It's described as being "lightly performed" along with "beautiful" and "mystic". Plus, Lilligant gets this move, so there's precedence for non-Bug-types learning it. At the same time, though, it's probably the single best way to break Aria Forme Meloetta and shoot her straight up into Ubers tier. Give her Boomburst as well for maximum Uber power.
Reasoning behind Lunar Dance: Nothing really. Probably the most out-of-line for her to have, given that it
is a signature move.
Also, if we're removing Serene Grace from Meloetta, then Relic Song's secondary effect rate should be raised to 20%.
Pirouette Forme should possibly have an ability other than Soundproof. What that should be I don't know, but Intimidate, Iron Fist, Limber, Quick Feet, Scrappy, and Super Luck all seem like decent choices. I'm personally leaning towards Intimidate, Iron Fist, Quick Feet, or Limber. Intimidate in particular would give Meloetta a unique trick, as Aria Forme Meloetta would be able to trigger Intimidate any time she wanted by switching to Pirouette Forme. She could then U-turn out and come back in later directly in Pirouette Forme to Intimidate a foe again. (Also Intimidate goes with the whole "fear the power of dance" style I've been going for with Meloetta for her other Pirouette Forme moves.) Limber would represent her natural agility and boundless, hyperactive energy in Pirouette Forme, as she
is constantly dancing. Quick Feet is along the same lines as Limber. Iron Fist is just for raw smiting power in a small, cute package.
Sky Uppercut is Super Effective on Flying-types. (Reasoning: Right now this move is useless. Also, given that its specific purpose is already to hit Flying-types despite them being in the air, this seems like a logical step up after Freeze-Dry was introduced.)
Double Kick has 40 base power. (Reasoning: Make it do actual damage.)
Zen Headbutt has 100% Accuracy and a 30% flinch chance, keeping in line with Iron Head. Headbutt has 80 base power, keeping in line with Iron Head.
Dizzy Punch has 75 base power, keeping in line with the Elemental Punches.
Disarming Voice has 60 base power, keeping in line with Swift and variations thereof.
Moonblast has 90 base power, keeping in line with Thunderbolt and company.
Flame Charge thaws out the user if they're frozen. Reasoning: You light yourself on fire and charge into the foe! Why wouldn't that thaw you out? Especially since both Flare Blitz and Flame Wheel already thaw the user for the same reason.
Searing Shot thaws out the user if they're frozen. Reasoning: The fire from this move goes out in an uncontrolled blast in all directions, fired from the centre of the user's body. If an instantaneous omnidirectional blast of raging flames centred on
yourself doesn't thaw you out, what would?
Slash, Psycho Cut, and Night Slash are buffed to 90 base power to match up with Leaf Blade. Slash and Psycho Cut are nerfed to have only 15 PP at base, matching up with Night Slash and Leaf Blade.
Latias gets Illusion as her Hidden Ability.
Reasoning:
— Sapphire 'dex entry: LATIAS is highly intelligent and capable of understanding human speech. It is covered with a glass-like down.
The POKéMON enfolds its body with its down and refracts light to alter its appearance.
— Fire Red/Leaf Green 'dex entry: It can telepathically communicate with people.
It changes its appearance using its down that refracts light.
Note: You'd have to fix the behaviour of Illusion + Mega Evolution on Showdown. If I recall correctly, right now, Mega Evolving while under an Illusion causes Showdown to display the sprite of your pre-Mega form instead of your Mega Evolved form. (Then again, I might be misremembering and it's actually only Imposter + Mega Evolution that causes that particular behaviour. I know for certain that it happens with Imposter, however.)
As for Latios, obviously if Latias gets a Hidden Ability then he should get one too. As for what that should be, my best guess is either Unnerve or Intimidate.
Reasoning:
— Ruby 'dex entry:
LATIOS has the ability to make its foe see an image of what it has seen or imagines in its head. This POKéMON is intelligent and understands human speech.
It could use its ability to make telepathic projections to frighten its foes by showing them horrifying images.
Another possible ability would be Speed Boost. Reasoning:
— X 'dex entry: A highly intelligent Pokémon.
By folding back its wings in flight, it can overtake jet planes.
I have a feeling that Speed Boost Latios could easily go straight to Ubers though, so I dunno.
Also, because both Latios and Latias are renowned for their ability to reach incredible speeds in the air, both learn Extreme Speed and Dragon Rush by level-up.
All Eeveelutions get Adaptability as a possible ability, inherited from an Eevee with Adaptability. Original abilities are gained from an Eevee with Run Away.
Bulletproof also grants immunity to shot moves (Mirror Shot, Gunk Shot/Dust Shoot, Mud Shot) and Cannon moves. (Flash Cannon, Hydro Cannon, Spike Cannon, and Zap Cannon.) It does not grant immunity to Parting Shot (Japanese name translates roughly to "parting sharp remark" or "parting retort") nor String Shot (Spit Thread in Japanese).
Pignite and Emboar are Fire/Ground. Enough with the Fire/Fighting starters already!
Dewott and Samurott are Water/Fighting.
Granbull gets Pixilate in place of Quick Feet, giving it an arsenal of Fairy-type moves to get abuse for powerful STAB. (In Gen VI, its entire moveset is still Normal even though it's now a Fairy-type, meaning it takes forever to get something it can actually get STAB on.)
New Ability: Dry Ice: This Pokémon's Weaknesses and Resistances are inverted. (NVE hits become SE, and SE hits become NVE.) This includes weakness/resistance to Stealth Rock. Immunities are unaffected. (For example, a Flying-type Pokémon with Dry Ice remains immune to Ground-type moves.)
Avalugg's Sturdy replaces Own Tempo on its ability list, and it gains Dry Ice as its Hidden Ability.
Articuno's Hidden Ability is now Dry Ice. Articuno's Hidden Ability is now released.
Edit: Also, this one's a bit of a stretch, but...
Dedenne's BST is now 77/78/67/101/77/111, 511 total
Dedenne's Pickup is replaced with Hustle, while Plus is replaced with Infiltrator.
Dedenne now learns Spark, Electro Ball, and Encore by level-up. It also gets Hone Claws and Dazzling Gleam by TM. Signal Beam is a tutor move for it. Nasty Plot is an Egg Move for it.
Reason: Because they make sense for an overaggressive little mouse thing. Also so it can actually hurt things in OU. Behold... THE POWER!
+1 252+ Atk Life Orb Hustle Dedenne Play Rough vs. 0 HP / 4 Def Multiscale Dragonite: 348-411 (107.7 - 127.2%) -- guaranteed OHKO
Protip: Don't let Dedenne boost if you value
not dying. Fortunately, 77/67/77 defences are still completely terrible, even if it does have the ability to hit like a truck on either the physical or special side after a turn of setup. Special-based Dedenne can actually attack before setting up, while HustleClaws Dedenne can't really afford to try to attack without setup thanks to Hustle. But if HustleClaws Dedenne gets its Hone Claws up, watch out! 111 base Speed is also a particularly trolly speed tier, so it has that going for it as well.
@OP: Shouldn't Water Shuriken be listed as changing from Physical to Special, and not the other way around?
The Reptile: If Kirlia doesn't get to Rapid Spin (list of rejected changes in the OP), then you can be assured that Miltank definitely shouldn't be able to Rapid Spin. Kirlia is far spinnier than Miltank ever was. Miltank
rolls, it doesn't spin. Rapid Spin = horizontal spin. Rollout = vertical spin, like a wheel.
Seconding giving Mega Beedrill either Megahorn or a buffed Twineedle, by the way.