
This thing needs
Magic Guard to be usable. Making your Clefable Stealth Rock weak just for slight boost to its stats and decent boost to SpA is not worth it, let alone using up a Mega slot.
The only alternative ability I could think of would be something that does Anti-Gravity, which would require coding a whole new field effect.

Thematically, Victreebel should get something similar to Gulp Missile. Its entire stick is that its oozing out poison and touching it cause more poison to spill.
Specifically if Victreebel is hit and has the move Belch, it will use Belch immediately. Since it doesn't learn any biting moves, I'm opting to not having any move requirements, which really buffs the ability. Call it
"Gag Reflex"

In terms of positives, Dragonite losing the ability to Tera makes Mega Dragonite more appealing. And honestly it's pretty hard judging what it should get based on design. Some potential abilities for it would be
No Guard or even
Gale Wings. There is also the possibility for brand new abilities. Like one that cause Charge/Recharge moves to not require a charge/recharge, or something that has to do with boosting Wind moves by 50%.

Meganium should have been a physical attacker with Flower Veil imo. A bulky Sword Dancer immune to Burns and Intimidate would be great.
But Special Attacker is fine I guess. Still immune to all status and stat drops from Snarl, Sticky Webs, random SpD.
I also feel like
Flower Veil would also be more viable than Triage because of that. Triage is nice, but without Calm Mind it doesn't sound as appealing as only Giga Drain and Synthesis get priority.

Give it
Sheer Force. We can pretend Skibidi Feraligatr doesn't exist and pretend its just using Sheer Force as God intended.

I feel like it'll get Sharpness in the actual game with new slicing moves, but
Touch Claws is good for now.

Snow Warning on Mega Froslass might be a bit strong.
100% accurate Blizzards
1.5x Def
(for reference: 252 Atk Great Tusk Headlong Rush vs. 0 HP / 0 Def Froslass-Mega in Snow: 177-208 (62.9 - 74%) -- guaranteed 2HKO)
Aurora Veil
All with Nasty Plot, 140 SpA, 120 Speed, Thunderbolt, and Shadow Ball.
But honestly Idk what else to put that fits.

Alternative ability being
Def Supreme Overlord, getting a 1.2x boost for every fainted ally. That would be better for Emboar's stats since its otherwise still slow.
Sand Rush or we riot.
Tinted Lens (or renamed Tinted Lens) or we riot.

I'd say give Scrafty
Shield Dust. The shed skin looks like a white Covert Cloak. Plus dead skin cells are part of dust too.

The Pokedex says it outputs 10 times as much electricity with its fake Eelektrike. So there is a bunch of ideas for abilities related to electric. Like making Electric moves bypass immunities, having Electromorphosis, Electric based Parental Bond, Electric Surge, etc. It's honestly looking to be dukie anyways with so much of its base stats being wasted to make it a SFMA.

The Pokedex mentions that Chandelure gets a special eye that lets it see our world and the after life at the same time.
SpA Supreme Overlord anyone?
My bad thought SO boosted only Atk.

Give it
Old Dauntless Shield. Dauntless Shield should have never been nerfed when Zacian was the problem in Gen 8 and Zamazenta did nothing wrong.
Alternatively, I always thought Intrepid Sword and Dauntless Shield should be like Download (and Zacian being a mixed attacker).
Defensive Download would also be a big boon for Chesnaught as it would appreciate either side being boosted but prefer Def when using Body Press.

Levitate would ironically be somewhat bad since Delphox would lose out on Expanding Force in Psychic Terrain, which would be big for it. It does make the most sense thematically at least.

Battle Bond is a bit much, don't you think? It gets Nasty Plot now and you want +3 SpA +1 Spe Mega Greninja running around?
It doesn't even fit since Battle Bond was all about Ash and his Greninja's bond.
Mega Greninja should instead get something related to that stealth aspect. Being that
X ability causes the first time Greninja uses a move to deal 1.3x boost. Kind of like Stellar Tera but better since you if you use Hydro Pump then Water Shuriken you still get the 1.3x boost to Water Shuriken if it was your first time using it.
idk what to call it thought.

Give it
Moxie as a joke. ZU will thank us with such a strong Mega.
Adaptability or we riot.
Reckless or
Rockhead (or a renaming of those abilities) or we riot

Give it
Suction Cups as a joke. What ever is below ZU will not care because it's that horrible.
Skill Link is a fine addition. Rock Blast after a Shell Smash sounds solid. I can't think of any other fitting ability.
H ability that gives it Focus Energy apon switching in.
Stamina could also work since promotional material mentioned how much bulkier it was.
D ability that causes moves affected by Weather to activate like they are in the respective weather. Thunder and Blizzard always hits, Aurora Veil always activates without fail, Solar Beam is one turn, ect. In the case of Weather Ball, it's doubled in power but is Normal type.
If that's too much,
Drizzle is probably the best and most fitting option.

I feel like Falinks would do the exact opposite since its multiple Pokemon that become 1 Pokemon. it should have
F ability that's Defiant for what stat gets lowered. If Atk is lowered, +2 Atk. Def is lowered, +2 Def, and so on. Same rules so no Close Combat giving you +1 Def/SpD. This would synergize with its tool kit better and frankly not be as stupid to fight against with +3 from using Trailblaze and Iron Head having better odds to flinch you than with Serene Grace.