alright im 99.99% sure that frankie will join. the reason? EVERY character that has joined so far has had a flashback. zoro had his childhood promis, sanji had his past with sef, usopp had the flashback with his dad leaving, nami had a flashback with her mother and arlong, and chopper had his flashback with dr hilruck. the only member whos questionable is robin and she hasnt officially joined yet. Franky has had a flashback and is the only one out of the people in this arc to have one.
However who knows, maybe paulie will have a flashback :P thats my theory...u can disagree with it if u want