It 100% has to be Kaato or Kaato + Jobin.I think Araki's bluffing, but if he really is planning to finally roll out the true villain of Jojolion, I'd be pretty bummed. Unless this part's going for another five to six years - which I don't think anyone wants to see - it's too late into the game for that shit. It has to be a character already on the table.
Remember Kaato was properly introduced only a short while ago. She is the final villain introduced late in the story.