I'll be honest, I've been playing this metagame non-stop for the past two days since I've been stuck indoors due to illness and as much as I enjoy seeing people's opinions on giving megas new abilities or reworking older megas, that should not be the focus at this current moment in time. I believe it's yet again time for some tiering action because I know this format can be in a better place if we aggressively start tiering like we did when we banned Mega Blastoise, Mega Alakazam, and Mega Kangaskhan.
These Pokemon should go ASAP


(mega)

I don't really get the anti-zygarde sentiment explaining that it only has 100 attack, that clearly didn't stop it in SM and SS OU, where those two tiers have the current counterplay this format + more, which includes an Intimidate Landorus-T and bulky waters like Tapu Fini. Maybe Zygarde feels manageable because we're all spamming unaware clef + slowbro cores, of course Zygarde would feel balanced. Try and build a team without those two and let me know how it goes, because outside of that, you're relying on Gourgeist-XL. Everything that has remotely made it past 1400s+ doesn't like this guy, and for good reason.
Mega Greninja on its own perhaps wouldn't be that strong, we do have special walls that can check it, for example Clefable but Clef needs physdef to check Zygarde, and now you see why Clef + Slowbro cores exist. Pretty much every MGren check is a Zygarde victim and I wouldn't even consider Zygarde the main offender here. MGren is just faster than most of the tier, has access to Nasty Plot, and it seems like whatever ability it has, it just seems to be annoying. Technician didn't seem that egregious until it starts using Tech boosted Flip Turn off of 125Atk to be the most annoying pivot in this tier, or spamming Water Shuriken and basically dealing with its offensive counterplay aka scarfers if Shurikens want to land more than 3 times. Bye bye to smth like Scarf Krookodile or Mega Aerodactyl and Mega Beedrill.
Mega Metagross just hits too hard, not particularly surprising when your STAB goes from Meteor Mash to maxed out Heavy Slam and Psychic Fangs is notably stronger than Zen Headbutt. Way too bulky for its damage output as well, how can Hoopa-U fail to OHKO this thing with Hyperspace Fury, it's a darn roll to kill that isn't in your favour. Almost crashed out in real time when it didn't kill. Also just makes teams even more reliant on Slowbro and Skarmory but if you face PuP Metagross, Slowbro just gets folded by Thunder Punch and so does Skarmory. Really the only thing that really holds it back is like Mega Delphox and Mega Greninja have type advantage but the latter can't even OHKO Metagross from full HP.
Gallade is in the same exact situation as Metagross, hits way too hard. Skarmory can at least have a semblance of checking Mega Metagross but Gallade? Sacred Sword just deletes it, leaving Slowbro as its most reliable check, until it clicks SD and hits it with Night Slash. ausma already spoke about Mega Gallade so I won't drag it on any longer.
Borderline

(mega)

(mega)
Mega Starmie is yet just another addition to the power level of this tier, which is leaving defensive play in the dust while offering not much other than high speed and raw damage output. Water is a pretty spammable STAB option and it ends up just either OHKOing most offensive threats it goes up against, or it just spams Flip Turn to get those mons in range the next time it comes in. It won't OHKO smth like MMeta but it will 3HKO with Flip Turn and rolls willing, OHKO you the next time with Liquidation. Offensive checks include the rare Sucker Punch from a bad mega and Mega Mawile, Mega Greninja, which is high in usage, Jolteon and Mega Manectric, and scarfers like Hoopa-U and Krookodile from what I've seen and experienced. Defensively, Slowbro would've been sound if they weren't running Thunderbolt and Volcanion too since you block Water moves but still take a million from Tbolt. Hisuian Goodra resists your STABs + Tbolt but you're like the easiest mon to wear down ever and you always just Flip Turn.
Mega Barb is lowkey kinda cheap, retaining Tough Claws with 140atk + Shell Smash is just a bit much imo? Despite the below average defensive typing, Barb has enough defence to find setup opportunities and after it sets up, what's the counterplay again? Oh right, the Unaware Clef and Slowbro, who would've guessed? A lot of people still trying to make screens work and it's mainly because of this guy, because everyone knows the moment it sets up, it's probably going to claim a kill. No scarfer outruns it, and your priority options are down to Aqua Jet and Bullet Punch, MMeta struggles to slot Bullet Punch (you have so many options) and Starmie is also struggling to slot Jet too. It certainly can, but dropping Flip Turn or Thunderbolt is lowkey trolling. There's also Water Shuriken Greninja, if you're actually running that.
Outside of the mons, I've mentioned, everything is somewhat alright but I would hope to see something happen to some of these guys. I don't really want to play another day of just having clearly broken mons around when we can do something about them so it begs the question, who's gonna be handling tiering moving forward? I like this metagame and I want it to be good as it can be, and slow/non-aggressive tiering is going to leave players dissatisfied. No problem if say, Zygarde and Mega Gren get the boot today for example, but I would like to have a path/timeline of when to expect some changes.