
X
Ability: Galvanize
Reason: Well besides no custom ability when we have 5 mono electric Megas that have only a small pool of thematically fitting abilities, Galvanize comes to mind to help Raichu X as much as possible.
Galvanize helps with Raichu’s sparce STAB options being basically just Volt Tackle. Raichu learns several normal moves, including the excellent Normal move Extreme Speed.
And looking at Raichu X’s stats compared to Y, Galvanize is a real possibility since Y otherwise outclasses X by a lot.

Y
Ability: Motor Drive
Reason: Honestly Raichu Y has some fantastic stats and to balance them out, it should be some filler ability like Motor Drive. It has the better stats and access to stuff like Nasty Plot and Grass Knot. Between the 2 it should get the mediocre ability.
Ability: Levitate
Reason: Levitate is far more useful for Mega Chimecho now, allowing it to avoid ground moves and Spikes.

Z
Ability: Super Luck
Reason: Did you see this things fucking stats? It doesn’t need any help.
Ability: Tough Claws
Reason: Need I say more?

Z
Ability: Sand Force
Reason: It sucks that Garchomp loses its Ground typing. However they made Garchomp Z a special attacker with the same speed as Pheromosa and gave it Nasty Plot. With that it really doesn’t need a fantastic ability either.

Z
Ability: Inner Focus
Reason: Let’s be real. This thing is getting banned immediately. They decided to give the Z Megas all minmaxed stats (and lied about Lucario Z’s stats) and made Lucario Z the most busted one. 164 SpA with Nasty Plot, Pheromosa Speed, Vacuum Wave for anything that’s faster, and amazing special coverage.
Honestly considering giving the Z megas hindering abilities since they otherwise tear up the Meta.
Ability: Earth Eater
Reason: It fits Heatran both competitive and thematically as well. While it doesn’t ignore spikes, it still makes Heatran immune to Ground moves and heals Heatran too, and Ground is its biggest weakness.
Thematically, it has a giant gapping mouth like when their wife’s boyfriend buys them a brand new Switch 2.
Ability: Compound Eyes
Reason: Darkrai turns into fucking Monoculus from TF2.
And since we can’t make custom abilities so that Darkrai shoots out crit rockets. This, along side its Pokedex mentioning how foes can’t escape from it could mean either it gets a trapping ability or an accuracy buffing ability.
Compound Eyes sounds the most reasonable ability and helps Darkrai a lot. While Sleep moves are banned, Darkrai has a bunch of innaccurate moves. Blizzard, Focus Blast, Thunder, Will-O-Wisp, and Thunder Wave. This gives them all at least 91% accuracy and makes it so the rest always hits.
Also Dark Void is now an attacking move that heals you.
Ability: Adaptability
Reason: I’m a bit bias. Golurk is my favorite. So glad they didn’t give Mega Golurk fuck ass stats like Malamar or Scovillain. I was watching Blunder’s stream and they suggested giving it Adaptability. It definitely looks like it will get a brand new ability with its design showing how its soul is bursting out. Something like physical and better Berseker, or Punching Triage. However no custom abilities.
Adaptability however is pretty good for it. A x1.333 boost to Earthquake and Poltergeist would be amazing, and it already gets Close Combat so it doesn’t need No Guard as much.
Really wished they gave it even a little more speed. Literally 6 more Speed would make Rock Polish outspeed those 151 Spe Mega and Mega Greninja with Adamant. But of course Golurk definitely needs more SpA.
Ability: Psychic Surge
Reason: Since this Mega basically combines both Meowstic, it’s in a way like Espurr which talked about its untapped Psychic powers. With Psychic Surge, it emulates that idea since all its powerful psychic energy is spilling out. It would also give Meowstic a niche that seperates it from all the other fast special offense Megas.
And also Expanding Force too.
Ability: Contrary
Reason: Ok hear me out.
Contrary here is more for gameplay. Crab here actually makes excellent use of Contrary with its signature move Ice Hammer, as well as the many stat dropping moves it has with Superpower, Hammer Arm, and Close Combat.
My flavor reasoning is that Crab gets longer legs but somehow is slower, which is CONTRARY to what one would think would happen.
Ability: Guard Dog*
Reason: Mechanically Guard Dog makes the most sense for Mega Golisopod since its previous abilities were about Golisopod being forced out. Now it flips the script by being unable to be forced out. Additionally it gains +1 Atk when intimidated.
Flavor wise…Golisopod - olisop = doG

Banned
Ability: Lightning Rod
Reason: Similar to Raichu Y, it has some pretty nice stats already. It doesn’t need much to succeed, and Lightning Rod does help mixed and special sets a bit.
Ability: Grassy Surge
Reason: Ok this thing is gonna be ass no matter what unless it gets some broken ability. At least with Grassy Surge, it can abuse Grassy Glide and sets itself apart from Rillaboom with its STAB Fire coverage.
And on the subject of Terrains give

Electric Surge and

Misty Surge so that there are 4 Megas with Terrain.
Ability: Merciless
Reason: Merciless would pair extremely well with Glimmora since it sets up Toxic Spikes or poisons Pokemon with Corrosive. It also looks way more aggressive like its in an attacking state.
Ability: Storm Drain
Reason: There is pretty much nothing flavorwise (ironic) for Tatsugiri that wouldn’t be a custom ability or buffed Commander.
Ability: Intrepid Sword
Reason: It literally has a giant sword. In fact that sword is definitely bigger than Zacian’s. Also it definitely needs a different ability to set itself apart from its base form since it otherwise would prefer to run Boots or Loaded Dice.