Isn't Jinbei too strong to join? I always thought that Oda had mentioned that the "Monster Trio" would always remain the same (i.e. The 3 top fighters of the SH crew would be Luffy, Zoro, and Sanji). Not sure about Luffy but I think it's safe to say that Jinbei (and likely Hancock) is stronger than both Zoro and Sanji, considering previous encounters with Shichibukai.
TBH, If it's gonna be someone we've already met, Perona does seem more likely than anyone else (maybe Marguerite to keep an eye on Luffy for Hancock, but that seems like a stretch).