Thoughts from a washed Pokemon player with WAY too much time on their hands (whenever the M-B VR drops):
Rises:
Mega Mawile: UR -> A+; An absolute beast in Trick Room, of which not many people have found the correct answers to prevent TR yet. Even outside of TR, there's only a handful of Pokemon that are even remotely equipped to take a hit (Corv fell off; Mega Aggron is unreliable), and Hippowdon is the only one that can do that while underspeeding Mawile if TR is active and can OHKO with correct investment. Steel/Fairy as a type combo in this format is also just so defensively strong; resists/tanks/threatens many of the M-A threats (Meow/Basc/Lop/Sneasler/etc.), and Intimidates the rest of the physical attackers it doesn't. Such a massive teambuilding headache that makes it the best mega in the format right now.
Mega Metagross UR -> A; Missed this mon; still the absolute powerhouse as it ever was. Strong enough to be a top 3 Mega right now, but linear enough to not be better than Mawile. 4 Attacks great as is, taking your pick of BP/MM/EQ/TPunch/Ice Punch/PsyFangs, though I've also seen Rocks occasionally for compression and Hard Press > Meteor Mash for better power (for those that miss Heavy Slam). Has the ability to overtake Mawile sooner than later, but a great place to start out is in A.
Regular Swampert: UR -> B; Regular Pert is good enough on non-rain variants to be a great defensive pivot against the above megas + non-GK Mega Raichu Y's, and is just a generally good wall/compression/support. Very good for skill expression; might see more regular than mega.
Mega Swampert: UR -> B+; Mega Swampert is a different story. Unlike Mega Metagross, there's no guessing with Pert. It's going to be EQ/Wave Crash/Flip/Ice Punch, and if it isn't you've lost so much power and pressure. It's also overly reliant on Pelipper for setting rain and to be the Grass check. In games where Mega Pert isn't paired with Pelipper, it outright loses to Pokemon that it should counter because of coverage (GK Zard Y/Delphox/Raichu Y) simply because there's no weather. I've never seen a Mega require another pokemon so heavily, that simply fails to do its job without it. Not a hater of Mega Pert though, but it feels very conditional to build with; not as useful as people are giving it credit for.
Regular Blaziken: UR -> B+ Mega Blaziken: UR -> A-; Both do p much the same thing; Protect for Speed Boost, click attacking move afterwards. Regular LO Phyisical Blaziken is cool enough for those that don't want to commit to Mega, and those that do want SD Mega Blaziken will feel this thing hit like a truck. Both variants are better with screens support and aren't overly conditional; pretty easy to play without much thought.
Mega Raichu Y: UR -> A-; Fun Mega to use, annoying to play against. No Guard Zap Cannon/Focus Blast is such an insane combination, and the flexibility between Alluring Voice/GK/NP/Surf/Protect is enough to keep people guessing. It's the definitive glass cannon of the format, and we're pretty happy with it.
Mega Raichu X: UR -> C-; lmao. Someone will use this thing somewhere, but the fact that the only Pokemon that works with it gets invalidated right now because of species clause says a lot about how flawed this Mega is. Poor guy.
Mega Barbaracle: UR -> B-; My personal sleeper pick. If you haven't played Shell Smash Mega Barb yet, you should. Tanky enough with better typing at base value to Shell Smash as regular, better to mega after for the type shift. Better with screens. Plenty of time for this mon to be better, and simply isn't right now, but it will be better as more mons get added.
Mimikyu: B+ -> A or A+: Biggest glow up for Regulation B. Easy to splash on TR teams as support, and LO with Ghost STAB is beneficial with the new additions. The lack of multi-hit move and Taunt users in format enable Mimikyu to do whatever it wants and get away with it considering Disguise. A top tier threat that needs appropriate recognition.
Dragapult: C -> B- or B: Criminally low in M-A; needs to be higher for M-B. Set flexibility between DD/WispHex/Specs/Band or general support/utility is necessary to abuse and great to splash on. The only Pokemon in format that will be faster than everything not scarfed or Mega Greninja that also ignores screens a la Infiltrator. Particularly great against this new set of additions.
Rotom-W: B- -> B+: Very good against the new drops. Best, reliable physical wall to Mega Gross/Mawile/Swampert/Blaziken, and VoltTurns with Scizor pretty well; hard to effectively prevent. Useful as a generic physical wall/pivot with lefties/sitrus but also usable as a Trick Scarfer to spook Mega Zard Y and Basculegion.
Regular Greninja: UR(?) -> A- or A; Why is Mega Greninja on the VR and regular Greninja isn't? Regular Gren still puts up crazy offensive mixed pressure on most everything not faster, and flexes its insane movepool to boot. Faster scarfer than Scarf Meow/Basc/Chomp/Hydreigon, naturally faster and can OHKO Mega Metagross/Sneasler/Zard Y/Starmie/Volcarona/Staraptor, doesn't trigger Toxic Debris and can counter hazard with its own Spikes/TSpikes, and can run a variety of sets to compress on teams. Just because it doesn't have an auto-crit STAB move, doesn't mean that Gren isn't useful, and I'm always forced to guess the set every game I see one. We gotta respect the goat's name, man.
Drops:
Mega Gengar: A+ -> A- Became a worse Mega simply because of what was added. Can't really OHKO the things its faster than, but isn't faster than the things it can OHKO, except for Mega Gross/Froslass. WispHex + Dbond is probably the more ideal set for this atm, but is worse than Dpult for this set considering it's a better check to Blaziken/Chomp/Hydreigon. Feels more restricting to use and build around versus other Megas, but not so bad that it's no longer usable.
Mega Floette: A -> B+: Same boat as Mega Gengar; became worse simply because of what was added. More predictable than Mega Gengar though, and Mawile is the better Fairy of the format. Doesn't help that the majority of notable, new megas were either Steels, Physical attackers, and/or both.
Corviknight: A+ -> B; This thing sucks now. Takes too long to set up ID + Body Press against Mawile/Megagross/Pert, and outright loses to Froslass/Raichu Y/Blaziken. With the power creep that's been added, it's more worth using Skarmory for hazard utility/phazing, and debatably does ID + BP better with a speed tier that matches Mega Mawile's.
Debatable topics:
S-tier Pokemon: I'd like to think that currently, only Garchomp is the only mon currently worthy of S-tier, and Meow/Arch/Basc should be A+. Meow can function on its own, and is so versatile to the point where it might still have a case to stay S-. Basc is only viable because of Last Respects, of which more counterplay was introduced, and Archaludon's perfectly in-between the previous two. Not as linear as Basc, not as versatile as Meow, but naturally annoying enough with Stamina + Sitrus Berry to require teambuilding consideration. I don't have a particular preference on this matter, but I don't think they should remain in their positions going into M-B.