Ah, so we're finally getting our "familiar but not so similar" kind of battle spot special, sorta like that one time where Pokemon had to hold berries, only crazier!
Here are just my initial thoughts.
- The lack of Choice Scarf and Focus Sash is pretty huge for sweeping. So lets say you set up Dragon Dance with Dragonite: You don't have to have thoughts like "I hope that Garchomp doesn't have a Choice Scarf or a Focus Sash" behind in your mind, so it'll be easier to sweep with less information than usual.
- According to Serebii, item clause is still in. I wouldn't be shocked if this led to more Pokemon running Z-coverage moves instead of just STABs. Not only does this get around item clause, but since you can't run anything else anyways, I think the situationalness for using Z-coverage moves has become much less of a detriment. Think more stuff like Fightingum Z Tapu Lele (screws up a lot of Steels).
- Ironically, I think forcing Z-crystals onto everything actually lowers the power level of the metagame quite a bit, since outside of the initial nuke, you can't rely on stuff like Life Orb or Choice items for extra power. I don't think this will lead to stall being super common (though that could get annoying depending on how the metagame develops tbh), but I do expect bulky offense to be emboldened by this and the lack of Focus Sash.
Now, lets look at the Pokemon I'm most interested in!
Mew: I mean obviously we gotta address the elephant in the room: For the first time in battle spot history, Mew is allowed in a metagame where it isn't overshadowed by much scarier, Uber titans! While Mew's stats aren't particularly amazing now a days, I think Mew's crazy ass movepool could potentially make it a much bigger threat in 3v3 than the usual 6v6 metagames its allowed in!
See, Mew sorta reminds me of how Silvally (and Arceus in Special Season 2) is: you see it at team preview, and can't really tell what its going to do until it has a chance to catch you by surprise. Now, its not totally the same as Silvally, since Silvally changes typing, and tends to run similar movesets across its types. There's also the fact that Silvally is a very rare and niche Pokemon.
Mew on the other hand, is probably going to be everywhere because it being allowed is a big selling point of next season. However, I think Mew is gonna be more than just experiencing what I like to call "the Ash Greninja" effect (being used often cuz you can use it for once). Mew's crazy movepool makes it capable of a ton of roles: it could be a good defensive Pokemon with Will-O-Wisp, it could a reliable setter of Stealth Rock, or it could be a special sweeper with Nasty Plot that can Baton Pass its boosts. There could be crazier gimmicky sets around too such as OHKO sets (if they allow RBY transfers, which is a big if I guess), Z-Transform sets, or Swords Dance Pass sets.
On their own, I don't expect these Mew sets to be super scary and metagame defining. However, I think with these diverse roles on ONE Pokemon, Mew suddenly becomes much more dangerous than usual, since one wrong switch in could spell doom in the fast paced nature of 3v3. For example, say you switch in Aegislash expecting a bulky Mew to shut down something, only to get shocked by a Nasty Plot + Baton Pass to a sweeper!
I think its hard to say what the "standard Mews" will be yet, but I expect Mew's versatility to be a crucial point in a lot of matches in this metagame!
Oh, one last thing before I go on to other mons: Obviously there's Mewnium Z to think about. Genesis Supernova sounds like a neat move for screwing with terrains a bit, and it gets around item clause pretty well if you want Psychium Z on something else such as say, Xurkitree. That being said, there are certainly more powerful options for Z-move nukes out there, so I wouldn't be shocked if Mew didn't get to use it that much because stuff like Dragonite were using their Z-moves instead. Because of this, I wonder if coverage Z-moves are a good idea. For example, with +2 SpA, Firium Z Mew actually OHKOs Aegislash!
Pheromosa: I agree with
Lionyx about Pheromosa, the lack of Scarfs is pretty damn good news for it! The only thing that will be outspeeding Pheromosa without priority moves is Speed Boost Blaziken, and even that's outsped if you don't care about power loss and use a +Speed Nature on Pheromosa! In fact, I think Naive could come in handy more than usual, since this also lets Pheromosa outspeed +1 Dragonite and Gyarados (I know Extreme Speed is an issue in the case of Dragonite, but we could always pair Pheromosa with Tapu Lele lol) I wonder if more people will use the special set as well with Fightnium Z being a much easier option to consider.
Dragonite and Gyarados: I grouped these two together because I expect these two to be the best speed boosting sweepers in the format, thanks to basically only being outsped by Naive Pheromosa! Dragonite in particular sounds killer because the lack of offensive items is gonna make Multiscale quite a bit more effective. Gyarados on the other hand, should be a decent check to Pheromosa without the potentially rare Hidden Power Electric, and it has Taunt, which could come in handy against some slower Mew.
Mimikyu: With Mimikyu being the closest thing to a usable Focus Sash Pokemon, I expect this thing to STILL be pretty much everywhere, especially since its a good user of Z-moves already. It also can be a detriment to Mew switch-ins, though I'm not convinced its a surefire check when Mew could run Will-O-Wisp or Baton Pass with little to no problems. It'll also always outspeed Tapu Lele now, which is a big boon for it imo.
Blaziken: Blaziken is already a great user of Z-moves, but I think in this metagame, this will become even more apparent since there's nothing that really outspeeds Blaziken at +1 besides Naive Pheromosa!
Porygon-Z: I'm not sure if this will be super common, but the lack of Life Orbs and Scarfs sounds like great news to Z-Conversion Porygon-Z to me!
Pyukmuku: I don't expect this thing to be common in the slightest, but I have to admit: the lack of offensive items outside of Z-moves actually sounds like a dream to this thing lol, I wonder if more of those Z-Purify teams are gonna pop up...