Mega Absol should be Dark/Fairy. Everything else could remain the same, though I wouldnt be against losing 15 SpA and putting it in Speed or Attack. I also wouldnt mind if it got pixelate, but it doesnt really make sense for Absol and Magic bounce is useful at times.
Mega Houndoom should have been houndoom's evolution, and then that mon gets a mega. Game freak really dropped the ball on this one. But in terms of what it is now, I think it should've gotten Defiant, Moxie, or Adaptability as its ability, as solar power sucks no matter in the sun or not. A pokemon as frail as houndoom shouldnt be losing HP every turn.
Mega Garchomp should have had 10 BP less in SpA and kept his speed at 102. This I think is the main reason why it's not used ever: its too slow to warrant a Mega Slot and its defenses, while great, dont justify it being slower than most of the unboosted metagame
Mega Abomasnow sucks. But I have no idea how to fix it. Since gen 4, abomasnow was a flawed pokemon to begin with and the mega didnt really fix the problem. It can do well against un prepared teams, but how many games are you going to play in a row that don't have a talonflame? Even if they dont, i guarantee theres at least one fighting, steel, or poison type move that will render abomasnow useless
Mega Audino should have kept regenerator, that's the only thing I'd change. But even without it, Normal/Fairy is great. it's an amazing cleric. The only reason imo that people dont use it more often is that there are better fairies that fulfill the same role. In league play, its not a bad draft pick if your first mega choice was taken.
Not really trying to go through all the megas, but that's a good start lol