I like the idea of adding sheer force to steelix like its base form has
What if we gave Abomasnow ice body? It already has snow warning so it can activate itself. Honestly might not be worth it but a abomasnow just feels worthless currently
I really want sceptile to get something new but not sure what adaptability maybe? Idk
Other megas I think could get something new
Chesnaught stamina-seems interesting I think
Clefable magic guard or unaware-serene grace is just not worth it
Dragonite no guard, let it happen. Just take out dynamic punch and zap cannon, they haven't been part of its movepool since gen 2 so I think it would be fine
Excadrill infiltrator? It doesn't do a lot right now that base form can't do, at least this lets it deal with screens
Floette fairy aura or soul heart-not sure if these are actually better but may be more interesting
Glimmora adaptability, I said this before but rocky payload doesn't fit as it's meant to be more of a triple type kind of ability
Hawlucha moxie? Stamina is good but I think moxie makes more sense
Malamar simple? Not necessarily better than contrary but honestly Malamar is already really bad, at least simple nasty plot stored power could be funny
Scovillin chloryphill? At least it would be a unique niche now that heat rock is in the game. Maybe Orxhalcum pulse?
Scrafty intimidate or moxie. No idea why it still has shed skin
Staraptor is good with tough claws, but it's kinda boring since skarmory has it and such similar stats. But idk what would be better
Raichu x electric surge? Idk if this would be better or nor
Sheer Force Mega Steelix and Ice Body Mega Abomasnow sound terrible.
Mega Steelix gets Iron Head, Crunch, and Iron Miss as viable Sheer Force boosted attacks. It'd be a discount Excadrill anyways on account of it being able to either hit Rotom-Wash/Mega Heatran, or double its speed in Sand. And gets Rapid Spin. And doesn't take up a Mega slot.
Ice Body would only heal a total of 1/4th your HP at max, unless you use other Snow setting Pokemon, which ends up just being Slowking-G using Chilly Reception. You likely won't even get that 1/4th of your HP back since Abomasnow is slow with tons of weaknesses, so it'll be forced out a lot. You know what helps it stay on the field for longer? That x1.5 boost to Def.
Chesnaught should get Daughtless Shield, but Stamina is fine, if not a bit much.
Clefable should absolutely get Magic Guard. Going from hazard immune to SR weak is absolutely terrible. Becoming an arguably weaker Togekiss doesn't help either.
Dragonite I agree with.
Infiltrator is interesting for Excadrill, but we all know it should and probably will get Sand Rush. It only has Mold Breaker for balancing reasons.
Floette getting Fairy Aura would be overkill Fairy damage, but it doesn't help Floette getting past the Fairy resists. At least Regenerator helps since Floette can fire off Light of Ruin into anything, deal a good amount of damage to everything besides like Heatran, then switch out to heal.
Glimmora with Adaptability is kind of eh. A problem with Adaptability is when you have it on a Pokemon that has sucky STABs. Glimmora actually doesn’t use its Poison STAB all that much for offensive purposes (not counting Mortal Spin). So you end up with a weaker Mega since Rocky Payload provides a better boost.
Hard disagree with Hawlucha. Stamina is good on Mega Hawlucha and fits its flavor based on promotional material (then again promotion material said Mega Lucario Z would be more defensive).
Malamar would need Wonder Guard or some custom ability to be viable. Even with Simple + Contrary (yes it got that at one point), it still sucked. Mega Malamar suffers from bad defensive typing, being slow, and shitty offenses.
Mega Scovillain with Chlorophyll would just be a worse version of regular Scovillain. You do more damage with Life Orb without wasting a Mega slot, and regular Scovillain was already bad despite being the first Chlorophyll Fire type.
There is also no Drought users that aren’t Megas too, which is also another advantage regular Sc*villain has. “An advantage regular Sc*villain has” should not be a sentence uttered by anyone, yet somehow such a sentence would apply to Scovillain’s own Mega if it got Chlorophyll.
Flavor wise, I think Scrafty should get Shield Dust. Dust is partly dead skin, so it has a dust shield in a way.
An ability that would benefit Mega Scrafty however would be Regenerator. Scrafty got Parting Shot, so it can Parting Shot while healing itself. It only has Rest Shed Skin and Drain Punch as recovery, which as a more defensive Mega is what it needs help with. And shedding skin is kind of like regenerating it.
Mega Staraptor is hard to give an ability since GameFreak decided to make a bunch of Flying/Fighting types and made them all nearly the same, and made Mega Staraptor have extremely similar stats to Mega Skarmory (though Mega Pyroar is really close to Mega Houndoom and Mega Garchomp is close to Kyurem-Black, so it’s not like its a new thing)
Tough Claws is kind of the generic buffing ability for physical attackers. Its basically Sheer Force for physical attackers.
Another less generic option would be Wind Rider. Not only are multiple wind moves super effective against Mega Staraptor, but it also boosts Atk.
Mega Raichu X should get Galvenize.
Yeah it already gets the +1 Atk from Volt Tackle, but that’s really all it does and Ground types prevent that boost anyways. With Galvinize, you get Electric E-Speed, which would let it hit the main stronger and faster Megas or just Pokemon.
Even if it means giving up that +1 Atk boost from Volt Tackle, it’d be better off with Galvanize Extreme Speed and Return/Double-Edge.
Garchomp-Mega-Z: Rough Skin 
mega is honestly pretty busted, I'm not sure if it's banworthy but its surely a really good mon. Insane speed tier + Great coverage and good sptack means it's only really checked by like clef and thats about it.
is also pretty good, its fun to consider the applications of competitive as well, you can switch it in on defog, then mega and basically anything gets OHKOed
, 
, 
, 
, 
Z, 
Y, 
, 
, 
, 
and 
(
too if you want)