I was just going to use the [Gen 8 Anything Goes] format, but to be honest, I'm not up to date on the meta.
Are you? Do you have any suggestions?
As for items, I was going to go with NO ITEMS, except for items awarded by winning battles.
I figured for the most access to Pokemon we'd use Natdex format. Though that brings into question what mechanics you'd actually want used, because it allows Z-Moves, Megas, *and* Dynamax if I remember correctly. It's possible we could ditch all three mechanics as to not pressure people into using specific mons, therefor the tournament could be seen more as a raw skill kind of thing in-lore, perhaps? That way you aren't pressured into choosing a Mega at all. (Though Megas can also help compensate for stronger mons chosen across the board. Regardless if we allow Megas, I have two that can fit into my idea for a character, so I'll make it work.)
So no held items in the beginning? Depending on what's awarded this could be a huge advantage to certain players over others. If these are all veteran players, I think it'd be fine for them to have held items. (And would have to, if we include Megas)