I love the 2.3 Tidal Wave Update The water physics, swimming, and custom move animations are really cool. Since Quick Match and public lobbies are already in the game, I think the next best additions would be a 2v2 tag team mode and a ranked matchmaking system with Elo leaderboards. It would be awesome if players could switch teammates during fights or use assist attacks, especially since the game already supports 4 players in modes like Mewtwo Raid and Soccer.
It would also be cool to see Gen 9 Terastallization added alongside Mega Evolution, plus an endless Battle Frontier roguelike mode that you can play offline. These additions would make the game even better. Take your time with the next update, even if it takes a few weeks. I'd rather wait a little longer for something that's polished and balanced!