Here are my extremely tentative early thoughts on the meta:
Necrozma-Dusk-Mane @ Weakness Policy
Ability: Prism Armor
EVs: 252 HP / 252 Atk / 4 SpA
IVs: 0 Spe
Brave Nature
- Trick Room
- Sunsteel Strike
- Earthquake
- Knock Off / Swords Dance
Necrozma-Dusk-Mane @ Life Orb
Ability: Prism Armor
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Dragon Dance
- Sunsteel Strike
- Earthquake
- Knock Off / Photon Geyser
Likely the two most common variants of Necrozma-DM and I think they will be some of the most dominating forces of the metagame. Time will tell though. I'd predict the Trick Room version to be more common as it's more ditto-proof and does better vs. opposing offensive teams.
Corviknight @ Shed Shell / Rocky Helmet / Leftovers
Ability: Pressure
EVs: 248 HP / 252 Def / 8 Spe
Impish Nature
- Body Press
- Iron Defense
- Defog
- Roost
Will once again be really common in this meta, I think. The reason being that I foresee Trick Room, Dragon Dance, and Swords Dance Necrozma-DM being a huge threat. As far as I can see, the best moves usable against Corviknight from Necrozma-DM is Knock Off, which is obviously limited. I see Iron Defense as being a must now, because Necrozma-DM can boost up his defense and attack. While a dynamax Necrozma with boosted defenses is obviously ridiculous, if you're just able to sit on him with Corviknight he will at least get pressured stalled and likely die eventually from boosted Body Press. Not only all of that, but he retains the same utility with defog and checking Zacian-C and Excadrill.
Reshiram @ Shuca Berry
Ability: Turboblaze
EVs: 4 HP / 252 SpA / 252 Spe
Timid Nature
- Blue Flare
- Dragon Pulse
- Roost / Earth Power
- Flame Charge / Earth Power
+2 252+ Atk Necrozma-Dusk-Mane Earthquake vs. 0 HP / 0 Def Shuca Berry Reshiram: 276-325 (80.9 - 95.3%) -- guaranteed 2HKO
Obviously not a reliable method of dealing with Necrozma-DM at all, but with how offensive the metagame is looking to be, I think doubling the offensive pressure with a secondary check to big threats will be quite useful, kinda like Shuca Dialga for Mega Mence in Gen 6. It would definitely give Reshiram some more utility.
Hell, it may even be useful to look into some Defensive EVs for this thing since it has some good resistances vs. Zacian-C:
+1 252 Atk Zacian-Crowned Play Rough vs. 8 HP / 252 Def Reshiram: 213-252 (62 - 73.4%) -- guaranteed 2HKO after Stealth Rock
+2 252+ Atk Necrozma-Dusk-Mane Earthquake vs. 8 HP / 252 Def Shuca Berry Reshiram: 218-257 (63.5 - 74.9%) -- guaranteed 2HKO after Stealth Rock
which still leaves you 244 Special attacks EVs + Modest Nature for some solid power, but maybe I'm thinking way too deep into it though lol
Mewtwo @ Leftovers
Ability: Pressure
EVs: Something
- Stored Power
- Aura Sphere / Shadow Ball / ???
- Cosmic Power
- Recover
This set could potentially be neat, I think. You have Pressure, Cosmic Power, and Recovery just like Eternatus, but you also come packed with Stored Power which is really nice. I was thinking of the Fighting move for Dark types that would be immune to Stored Power, but Necrozma-DM and Lunala are annoying, so potentially maybe losing out to Dark-types would be sufficient if getting the Ghost type hit on those common threats is worth it. I of course expect the Nasty Plot set to be the most common but yeah
Lunala @ Heavy-Duty Boots
Ability: Shadow Shield
EVs: 252 HP / 252 SAtk / 4 SDef
Quiet Nature
IVs: 0 Atk / 0 Spe
- Moongeist Beam
- Focus Blast
- Trick Room
- Roost / Psyshock / something
I expect something like this to be the most common Lunala set. Why even bother messing around with any other item besides Heavy-Duty Boots? Guaranteeing that Shadow Shield will activate is a huge thing, and with how offensive the meta is looking, Lunala's ability to come in on a lot of offensive threats and completely turn the momentum around is huge. Ghost and Fighting is perfect neutral coverage already, so I don't think you need to run Psyshock, but the ability to cancel out priority for Marshadow is really neat. Maybe Agility would be better but Trick Room seems really solid in theory.
Tyranitar @ Chople Berry
Ability: Sand Stream
252 HP / 4 Atk / 252 SDef
Careful Nature
- Crunch
- Earthquake
- Stealth Rock
- Thunder Wave / Rest / Stone Edge / whatever
A slightly modified version of the already used Tyranitar set seems pretty solid. This looks like it could be a good switch-in for general special attackers like Reshiram and Kyurem-White. With Chople Berry, you don't have to worry about even +1 Focus Blast from Lunala, and it gives you a chance at surviving a Life Orb Mewtwo's Focus Blast. This set also double in utility because you can set-up Stealth Rock and also Sand, since Excadrill looks just as potent as ever and the Sand has the bonus of making Lunala's shield obsolete and messing with Necrozma's Morning Sun. TWave seems useful for crippling Dynamax mons, but who knows how this set will end up developing.
Ferrothorn @ Leftovers
Ability: Iron Barbs
252 HP / 4 Atk / 252 Def
Relaxed Nature
- Knock Off
- Curse / something else?
- Spikes
- Leech Seed
Dragon Dance Zekrom looks like it can be really nasty, so I foresee Ferrothorn gaining some more usefulness as it can wall Zekrom pretty hard. Curse is so that he can't just Dragon Dance in front of you forever, it also helps for the potential Gothitelle. Spikes is still as nasty as ever. This set is also neat because it can under-speed the Trick Roomers of Necrozma-DM and especially Lunala, smacking them with a nice Knock Off. You also just sit on Defensive Necrozma, Tyranitar, and other things.
Necrozma-Dusk-Mane @ Leftovers
Ability: Prism Armor
EVs: 252 HP / 4 Atk / 252 Def
Impish Nature
- Sunsteel Strike
- Morning Sun
- Earthquake / Toxic
- Stealth Rock
Seems like a really solid utility defensive set, one of the few mons I see that could take on a set-up Zekrom, and it could probably even at least damage a set-up opposing Necrozma-DM. I'd think you need to run Earthquake more than you would need Toxic, which while unfortunate, lets you deal with Substitute Zekrom and opposing Necrozma-DM better. Toxic could have it's uses though, especially if CP Mewtwo does actually become a thing.
Zekrom @ Haban Berry / Leftovers / Life Orb
Ability: Teravolt
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Dragon Dance
- Bolt Strike
- Dragon Claw / Outrage
- Roost / Substitute
A really solid sweeper if it gets set-up, I believe. Substitute could be useful for certain defensive mons while Roost would let you run Life Orb if you wanted more immediate offense.
The Kyurems seems pretty meh so I won't comment on them.
anyway people don't be surprised if dynamaxing on these big boys gets banned soon so enjoy it while you can
edit:
252+ Atk Necrozma-Dusk-Mane Photon Geyser vs. 252 HP / 252+ Def Quagsire: 166-196 (42.1 - 49.7%) -- guaranteed 3HKO
Quag is still viable?
edit edit: Photon Geyser ignores Unaware so