That's indeed an excellent question and no doubt a mystery till the last moments. Pretty much the only thing that makes me wonder about Franky is his large crew or gang. They seem to really depend on him, and that makes me wonder if he's just going to leave them behind and join the Strawhats. In fact I feel that would be sort of counterintuitive seeing how Luffy views his crew more friends than subordinates. Actually fully friends and not at all just subordinates. I don't think he's going to insist Franky to leave his old bunch coldly to join a new crew.

Though if the World government is going to declare Franky a big criminal, that could of course happen...