I am always the one who screams that we shouldn't say "this" or "that" will happen and I'm not about to break my own policy.
Rather than a list of why Franky MUST join, this is a list of highly suspicious clues that are likely to result in Franky joining but it is true that ANYTHING could happen here. And boy do I mean anything.
Oda threw us for a loop last time, he rarely pulls the same trick twice (in a row at least)
Franky's VA is suspiciously highly popular with fans since his work as Jango and Bon Kurei.
In recent months Franky has been given an incredibly detailed history and a personality that grows stronger by the moment. It is very true that the same happened with Vivi as well though...
Franky has most recently been given a huge list of attacks. A very good sign.
Paulie is by no means a weak character in terms of strength or character, but he still seems somewhat plain as compared to Franky who is supposedly a rejected shipwright, his ability, despite a heavy learning curve for his place, seems to rival that of the only other shipwright taken in by the master Tom.
In relation to the last statement, we all know crew members have dreams, and Franky has already stated his. Most recently a clue from SBS would inidcate Paulie's dream was to become a great shipwright and he has already accomplished that. I'm not saying he couldn't have something else, but as it stands right now, Paulie is living his dream while Franky is working towards it.
Paulie seems to be creeping more and more towards the likes of a Wiper, a character who has a great reason to fight and can stand up for themselves, but in the presence of truly extraordinary individuals such as Luffy and dare I say Franky, seems somewhat dull.
Franky is super powerful but has two major weakness, a plus for Mugiwara Luffy crew members. If he was suited to be a background character, Oda could easily have given him the cola weakness and left it at that (similar to Wiper and the Reject) but two weakness gives Oda MANY wonderful opportunities to disable Franky or put him in danger later on.
Now, according to the above, which are merely observations, logic dictates that Franky has an excellent chance at being the next member. However, the most important thing to remember is, LOGIC DOESN'T MATTER HERE ^O^ In my opinion, it seems like Franky is in however OP sure as hell isn't up to me. I'm just as excited either way and I would love to be proven entirely wrong if by Oda sensei.