Welcome to Sword and Shield Anything Goes! This thread will be used for all things related to the new metagame - theorymonning, metagame observations, and set sharing are all encouraged! Since we're entering a generation with some pretty unprecedented differences, I'll clarify exactly how AG is going to work this generation:
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[*][B]No Megas or Z-moves[/B]: This should go without saying, but since both mechanics have been removed in Sword and Shield they will not be usable in the gen 8 metagame. Bye-bye Mega Rayquaza and Ultra Necrozma!
[*][B]Galar Dex and Released Pokemon only[/B]: This is probably the most significant change from last generation. No more Arceus, no more Primal Groudon, no more Xerneas, etc. In addition, Pokemon or movesets that are coded into Sword and Shield but aren't yet available ingame will not be usable until Pokemon Home releases or said Pokemon are distributed via events. This gives us a pretty limited pool of Pokemon to work with right off the bat, the metagame will likely be extremely centralized around a certain cover legendary in the meantime.
[*][B]In-game Restrictions only[/B]: As it's always been in Anything Goes, you're allowed to use whatever you want provided it's available via normal cartridge play. That means you can run OHKO moves and Evasion to your heart's content, but you still can't put Sheer Cold on a Yamper since it doesn't learn it normally.
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[B]New Pokemon / Early Metagame Observations[/B]:

[B]Zacian[/B]
This dog looks like it'll be the defining offensive threat of the generation. Most sets will use the Crowned Form due to its stellar Fairy / Steel typing and phenomenal speed tier. Play Rough and Close Combat together hit most of the metagame neutrally, giving it plenty of flexibility in the last moveslot on a Swords Dance set. 4 attacks seems like a viable choice as well given how strong Zacian is even without a Swords Dance boost - additional coverage allows it to bypass potential checks like Toxapex and Aegislash.

[B]Zamazenta[/B]
Unlike the other dog, this one looks terribly underwhelming. While it has superb stats in its Crowned Form and Base Form, Zamazenta lacks the tools it needs to take advantage of this. Without any noteworthy utility options, a defensive set seems underwhelming at best, while offensive sets pale in comparison to Zacian. It's possible a Howl set could act as a bulkier but slower boosting alternative to Zacian, but at the moment I'm not seeing much use for it.

[B]Eternatus[/B]
Eternatus seems like it has some potential. Dragon / Poison / Fire coverage is phenomenal as we know from Naganadel, and Agility with its high SpA stat could make for a decent cleaner. It has some other interesting tools in its kit as well - a bulky attacking set with Toxic Spikes or Recover seem possible as well. Overall it's quite hard to say how Eternatus will fit into the metagame, so it's definitely worth exploring more!

[B]Dynamaxing[/B]
Dynamaxing seems like a very interesting new mechanic. While the cover legendaries can't Dynamax, Pokemon like Excadrill or Ditto can take fantastic advantage of Dynamaxing's mechanics. It's still not clear how the meta will stabilize around Dynamaxing but it seems like it will require players to be adaptable and consider what the optimal Pokemon to Dynamax is and when to Dynamax it.

[B]Ditto[/B]
Ditto looks like it will be incredibly powerful in the initial metagame, since it's one of the most reliable ways of checking Zacian and Ditto tends to thrive in offense-heavy metagames. Ditto can also abuse Dynamax, which causes it to lose the boost to its speed from Choice Scarf but also gain the ability to switch between max moves.

[B]Excadrill[/B]
With Zacian dominating the metagame, Sand Rush Excadrill looks like a great option to revenge kill it, while also having access to hazar removal in a metagame with few viable removers. Excadrill also benefits from Rapid Spin's new mechanics - it can now boost its speed and remove hazards at the same time!

That's all - discuss away!