Really quickly I'd like to address this, because it's not 100% correct. Shell Smash only puts you at -1 Defense not -2, so the calc actually looks like this.
0 Atk Centiskorch Power Whip vs. -1 0 HP / 0 Def Blastoise-Mega: 248-294 (82.9 - 98.3%) -- 68.8% chance to OHKO after Stealth Rock
Now that's not bad at all and you could probably shift a couple EVs to attack to make that better, but it doesn't guarantee absolute removal of Mega Toise. There's also the inherent risk of Mega Blastoise just clicking Hydro Pump (which is somewhat rare but it is run frequently enough to keep in mind) instead of setting up, in which case this happens.
252 SpA Blastoise-Mega Hydro Pump vs. 252 HP / 172+ SpD Centiskorch: 308-366 (76.2 - 90.5%) -- guaranteed OHKO after Stealth Rock
meanwhile Power Whip maxes out at like 66%, and that's assuming you have had no chip done at all and are awake so you don't risk Sleep Talking into Fire Lash or something. Centiskorch might have some sort of defensive niche, but I wouldn't want the bug dealing with Mega Toise at all.
Now then, onto The Mega analysis Part Three, because I forgot a couple of actually really solid Megas and I felt weird about not having them.
Not really sure how I forgot about this thing the first two times, and it seems like ladder has as well. The Psychic Terrain nerfs really hurt the Psyspam playstyle that Mega Zam was often slotted on, and threats such as Genesect and Scarf Darm give it new headaches. But oh wow, you can't overlook this things positives. That speed tier is amazing once again, it has varied coverage options that let you decide what you want to beat, and it gained several new friends in pivots that can get it in safely. A great choice for a team looking for a decent late game sweeper.
(Latias) I've barely seen any Mega Latias and honestly I'm not really sure why. It has a very valuable offensive and defensive niche, most notably with a Calm Mind set which can take an exorbitant amount of hits from the Special Side while also posing a big offensive threat as well. Still a very solid Pokemon, although new metagame trends hamper it a bit.
(Latios) I've seen even less of these, probably cuz it isn't quite as good. It's mixed sets are still theoretically pretty solid though, especially since targets for its EQ like Heatran and Tyranitar are still present in the metagame.
Eh, I dunno about this one. I've kinda always been of the opinion that the amount of support it needs to do its thing isn't worth it, but it's hard to deny its potency if it picks up a Swords Dance. It also has a really tough choice between Earthquake and Close Combat in the last slot; CC allows you to hit things like Ferrothorn harder as well as preventing you from being totally stuffed by the Steel/Flying mons, as well as any Rotom forme that comes to call. Earthquake, on the other hand, hits Aegislash and Mega Metagross harder. A tough call that doesn't have an easy answer.
We're getting firmly into "Yes, you COULD use this, but why would you?" territory now. Mega Aero doesn't hit hard enough nor is it fast enough to justify a slot onto a team. That sounds weird because it has 150 speed and 130 attack, but its really easy to revenge kill as well as being laughed at by a myriad of defensive Pokemon that we have. And its weak to SR. Not a good combination of traits.
252 Atk Aerodactyl-Mega Earthquake vs. 0 HP / 0 Def Metagross-Mega: 158-188 (52.4 - 62.4%) -- guaranteed 2HKO after Stealth Rock
That's not good enough.
Only remotely usable on Trick Room, and I would much rather use Mega Camerupt, who's viability is questionable as it is. Sorry Ampharos.
This is a massive casualty of the anti-Dracovish effort, as a lot of things that beat Vish naturally destroy this. With proper support it theoretically becomes an interesting late game cleaner...although it really needs a huge amount of support because if it doesn't kill whatever is in front of it it's gonna get Revenge Killed. It could work on heavy hazard stacking teams with Zone support I suppose, but at that point just use a better cleaner and take the pressure off of the rest of your team.
Too frail, no super notable resistances, massive coverage issues, and not enough immediate power. Any questions?
Body Press is an interesting tool for a lot of defensive Pokemon, and Steelix is one of the biggest beneficiaries of it. Curse Sets could potentially put in some work now...except for the fact that Mega-Aggron exists and has an overall superior defensive typing and ability. Seriously, Ground/Steel is a fine typing on something like Excadrill but it really works against Steelix here. Aggron was already barely dealing with things like Darm but now you've got a mon thats neutral to Darm's STAB instead of resisting it as well as weak to Fishious Rend? No thank you.
A fast offensive pivot set that has the ability to push damage on a lot of things you wouldn't think it would be able to like Ferrothorn might look nice...but then you realize that Scarf Genesect exists and is just better while also not taking up your Mega Slot.
This was outclassed by Torn-T before, and now Torn can hold Heavy Duty Boots. Enough said, really.
This honestly could have been a somewhat interesting revenge killer and pseudo-wallbreaker but man that typing is rough. Its STABs are walled exceptionally easily and good luck finding a place to get in because that typing has some really exploitable weaknesses.
Lol
Altaria is naturally going to struggle in a metagame where the two best Pokemon are arguably Mega-Metagross and Genesect. Also Clefable still exists, and its weak to a lot of the new stuff. Not a good choice at all.
I guess you could use it as a Trick Room setter or something but like...Cresselia exists so I'm not sure why you would.
Last but not least we come to Glalie who...actually isn't the worst thing in the world! Ice/Ground coverage forces switches on a lot of Pokemon (especially with Freeze-Dry potentially in the mix) which gives Glalie plenty of opportunities to set up spikes. It struggles against Mega-Metagross and Genesect as well as a lot of new stuff but Spikes as well as a Refridgerate boosted Explosion means that this could potentially put in....some work, probably as strict Scarf Dracovish support because it actually threatens a lot of what beats it as well as laying hazards for it. I wouldn't call it viable, really, but at least it could potentially have...some purpose in the metagame.
Pretty sure I got all of them that aren't banned. Happy building!