So what do you think will be Gen 9's gimmick?

I personally don't think it will be another evolution/alt-form gimmick so soon after Gigantamax. I'm thinking it will be closer to Z-Moves where it will be some in battle attack. Maybe tag moves where once per battle you can use a combo attack while switching Pokemon. The Pokemon you are sending out to the field and the Pokemon going back in it's ball will do a powerful combo move.