What is your favorite Battle gimmick?

I would prefer that they gave older lines permanent evolutions rather than megas; I don't think there is anything wrong with some Pokémon gaining a fourth evolution. But if we can't give them that, I find megas far more interesting than protracted move animations, POKÉMON BEEG or a gaudy crystal hat.

Tera for me. It doesn't play favorites(any Pokemon can Tera), lasts until they faint and it applies Offensive and Defensive perks to the Pokemon in question without occupying an item slot or boosting the stats of the Pokémon in question. It can also make clever use of Pokemon abilities(Levitate for example) or simply bumping an existing Type by increasing the STAB bonus.