Act 3 is not ready to end, some thought that it would have already ended with the arrival on Onigashima but it may last at least 10-15 chapters if not more.
Some thought that the tragedy that happens in the third act was Oden's death in the flashback or Kanjuro's treachery but I have more and more the feeling that it will be the defeat of the alliance against Kaido and Big Mom instead and that the real final battle for Wano will instead happen in Act 4.
In addition of that I am even more certain that the final battle will happen in the mainland and that the people of Wano will so witness the defeat of Kaido and Orochi just like how the people of Cocoyashi, Alabasta and Dressrosa witnessed the defeats of the respective individuals who oppressed them : Arlong, Crococile and Doflamingo.
New actors might also come for this final battle such as Nekomamushi and whoever he convinced to come with him to Wano, the marines or the CP0 after the deal made with Orochi earlier in the arc, Hiyori, Tama and possibly other people like Weevil or the Grand Fleet.