If Mega Ability changes are being considered....

Mega-Y: I think this mon's ability should be nerfed to
Lightning Rod. I do not think the grounds in the tier are well-equipped to handle this Pokemon at all outside of running Scarf sets given its wide coverage with moves like Alluring Voice, Surf, and Grass Knot. That said, I most likely am in the minority of viewing this Pokemon as a potential issue - I'm curious on what other's think.

- So I will still maintain that this Mega's ability to ignore Trick is very nice on Calm Mind sets, but Serene Grace was also not the way to go for the ability imo. I really liked Kibo's suggestion of giving this Pokemon
Delta Stream, as that ability isn't overpowered, and would give some interesting trade offs to the Calm Mind set, canceling other weather effects, letting Clef leverage its increased defense to work better against threats like Baxcalibur without being weak to its STAB, and giving it better defensive MUs against stuff like Garchomp and Krookidile in exchange for the gamebreaking utility Magic Guard brings. That said, this is also a signature legendary ability specifically tied to Rayquaza, so I do not think it is likely Clefable will get it. It may be boring, but I think just giving it one of its base abilities in
Magic Guard or
Unaware would likely suffice - preferably Unaware since this would give it a cool trade off where it can more easily win Calm Mind wars at the expense of being vulnerable to status. I even think
Regenerator could be considered here.

- This Pokemon is cool and I think Soul Heart is an interesting ability that also fits it, but it is not enough imo for this Pokemon to work given that its Special Attack is a fair deal lower than other Megas and its speed is only adequate - with its coverage also not being all-encompassing enough to make up for these weaknesses. If we want to improve its offensive capabilities,
No Guard is the way to go, giving it perfectly accurate Hurricanes and Fire Blasts to nail most of the Fairies and Steels, with Zap Cannon being an additional cheese factor it can rely on to break through different Pokemon. If we want to take this Pokemon in a more defensive direction - either
Regenerator, Magic Guard, or
Natural Cure feel the most fitting - if a bit boring. I personally think Magic Guard would be the best since it would let this Pokemon act as a pretty decent Defogger with a set of something like Draco Meteor, Flamethrower, Roost, Defog - while also not being weak to pursuit. Watch out for Clefable though.

- Dragon's Maw is a pretty fitting ability on this Pokemon, but it could be better. Personally I don't want this Pokemon to be better since this format doesn't have the greatest water resist and dealing with a DD cheeser with 165 base attack seems annoying. That said, if we needed to improve it, just keeping its base
Sheer Force or giving it
Tough Claws to also boosts its Dragon STAB would be anough. I think Sheer Force + Dragon STAB is enough - even without the damage boost - since that nails most of the usual Water resist for good damage.

- There are talks about this mon being overtuned, which I am 50/50 on. I actually like Earth Eater on it to check threats like Baxcalibur and Garchomp. That said, if we needed to change it, I think just giving it the base
Flash Fire would be enough, though it would also just completely gut its usage since there isn't much reason to use it over base Heatran. Another interesting ability - that would also be a bit unlikely - is
Unaware which would give this Heatran a niche in completely shutting down various setup threats like CM Clefable, CM Magearna, and NP Darkrai.

-This mon is lowkey cracked as is, but has little reason to be used over base Excadrill because of oppurtunity cost. Mold Breaker is pretty fitting and a good ability already, but I think we can improve this Pokemon a bit more to make it worth using.
Earth Eater is a bit unlikely, but the effect of the ability makes sense on this mon given that its better at drilling in the Ground.
Tough Claws makes sense from a design perspective IMO as well - just look as those big Meaty Claws and would give this mon a greater edge in breaking through mons with Iron Head / High Horsepower.
Bulletproof is honestly very fitting on this Pokemon given its hard shell when drilling and would give it an amazing MU into key threats like Magearna, Gholdengo, and more - even moreso than base Excadrill.
Scrappy is very unlikely, but there is also merits in having a completely unblockable spin in this format - so it depends on whether we sacrifice realism for idealism lol.

Ice Scales is honestly fine, but realistically, I think this Pokemon is getting
Tough Claws, which I think would be the best ability for if it just gets used as a Trick Room attacker. Boosts Mach Punch as well.