Besides Luffy and eventually Nami, the crew really hasn't gotten new power ups since the time-skip (not including Haki). Here's what I think their next forms will be:
Usopp: Seriously, when is Usopp gonna use guns? Does he have a Batman rule against guns? He should start using guns and use the slingshots as a last resort.
Franky: I think he may use different fuel that will make him temporarily way stronger but insane. Not sure what tho. Maybe Sake?
Sanji: Sanji might just change out his shoes. Like, that can work right?
Zoro: New sword at Wano. Hopefully his eye is the personification of Asura and he'll use 12 swords or something.
Robin: Besides starting to use Armament Haki and awakening her DF, Robin doesn't need a new form.
Brook: Like Zoro, Brook might get a sword. Besides that, nothing new is needed.
Chopper: A new drug like Rumble Ball or the Energy Steroid. It might make him go back to being Feral like his Monster Point but for his normal forms. So he can be unpredictable but in his weaker forms so he isn't a big threat to the others.