I'm leaning towards either BB or Crocodile. I also assume that whoever he is working for will go after the ancient weapons which will be considered a declaration of war against the strawhats since both wano and fishman Island are territories Luffy claimed. With that being said I think a war with blackbeard will be more likely to come next since I'm expecting a race to laughtale between Luffy, Shanks, and Buggy with the world government trying to stop them.