Ok so M-Sableye for me is a given for A+. A cornerstone of stall right now, the premiere spin-blocker, extremely hard to wear down with the proper stall support, has the right utility to threaten various archetypes, and teams need to prepare for this at the level of an A+ ranked mon.
I'd like to see some reasoning as to how Dragonite can maintain itself comfortably at A rank at the moment. I simply don't see it besides being able to check offense a bit with banded extremespeed. I feel like it doesn't provide as much in the meta as it use to.
I made a mention awhile back when Albacore asked but Gliscor should drop to A-. It's got a lot of positives going for it on balance and stall builds with its defensive utility but dealing with M-Sableye stall is a pain for it and its become a bit of a liability with Greninja running about along with the increased viability of such threats such as Thundurus, M-Altaria, and Taunt/Sub M-Gyarados.
Tyranitar Mega for B+ not necessarily because the meta has got worse towards it but because we're now establishing opportunity cost between megas as a thing.
To add about Gliscor, being unable to properly stallbreaker against Mega Sableye stall beats the purpose of running stallbreaking Gliscor in the first place, one of his best sets. Other than being in desperate need of a Bisharp check (so no Lando-T), i don't see many reasons to use Gliscor atm, as Landorus-T is better against offense and Gliscor as a whole got much worse vs stall, so i think B+ is beter than A-.
I agree with everything else you said, and it's worth noting that Dragonite also got a big hit with the introduction of Mega Altaria, Mega Diancie, and Mega Metagross, all of which make the CB set way harder to use and also check DD Dragonite.
Moving on, here are the changes i would make:
Mega Lopunny: A ---> A+
Mega Altaria: A ---> A+
Mega Heracross: A ---> A-
Mega Slowbro: A ---> A+
Terrakion: A ---> A-
Mega Medicham: A ---> C+
Mega Lopunny to A+ is obvious in my eyes, this thing is one of the biggest threats to offense, can safely MEvolve thanks to great initial Speed and Fake Out, is great against balance with little support, and can even do work against stall with sets such as SubPuP, SubEncore, SubPass, and even Healing Wish lets wallbreaking partners have a second shot at messing up the opponent's stall teams.
Same goes for Mega Altaria, which provides an excellent blend of power, bulk, and longevity, and is also very versatile, with at least four viable sets that have lots of different options withing themselves. Mega Altaria's typing is crazy good in this metagame, and she has million tools at her disposal to take advantage of the free turns it finds.
Mega Heracross is still a very viable wallbreaker, especially against stall teams, but its matchup against balance and offense has gotten worse. Mega Altaria, Mega Gallade, and Mega Metagross are new checks that make Hera's life harder, Landorus-T is more popular than ever, and in general most offensive Pokemon have the upper hand against Mega Heracross one on one, leaving to Mega Heracross mostly defensive Pokemon to prey on. Also, for a MEvo that didn't get any better or any new niche, the even bigger opportunity cost hurts its viability a bit.
Mega Slowbro deserves to go in A+ solely because of its dual boosting set (Iron Defense + Calm Mind), which unless you have a Toxic user, NP Thundurus, Specs Electric-types (easy to take advantage of), TG Manaphy, NP Celebi, or Modest Mega Sceptile, is going to sweep you clean, not to mention that it's few checks and counters can be easily taken care of with a couple of teammates, or even by Slowbro itself, if he wears them down with Scald burns in the early-game. Plus, Mega Slowbro is one of the best checks to Mega Metagross and nearly every single physical attacker, which is saying a lot.
Regarding Mega Medicham, it's just outclassed by Mega Gallade, what else is there to say, and even C+ seems too generous.
Finally, Terrakion got one more check in Mega Sableye, which makes its life harder no matter how you see it. Mega Sableye counters Scarf Terrakion, checks Band sets, and walls SR + Taunt sets, meaning Terrakion has to resort to SD + SR sets if it wants to be a reliable SR setter and can't prevent anymore SR from going up against opposing SR setters. A ton more faster stuff that outspeed and KO it is also bad news, as is getting fucked by one of the most common Pokemon found on offense, Landorus-T.
Other changes i am thinking about are Manaphy to A, because he is a good wallbreaker that can counter both Mega Sableye and Mega Slowbro, Mew to A- (being useless against they playstyle you are supposed to help with sucks balls), and Mega Gyarados to A+, though i am a bit iffy about this because i don't know if Mega Gyara is as good as monsters such as Mega Lopunny and Mega Gallade on offense, so i want to hear more opinions about this.