Well, it's been a long time since I posted in this thread. I think it would be fun to try and write my thoughts:
My personal point of view when looking at a new crew member is looking at the dream and relationship with the crew alongside a few other personal criteria that don't matter that much in the long scheme of things but I think Oda would have to consider in a permanent member. The dream part is important because it needs to be finished at the end of the series, not before.
As a example, I don't think a fighter would be added at this point in the saga simple because Oda can barely deal with the characters as is. However I can see a gag or weak character joining, because he can easily add some scenes without taking much of a chapter, while the monster trio fights. That's one of the reasons why I personally think Franky and Brook don't appear too much compared with the others: their gags are too specific and as fighters they take a backseat to the others. This is also why I believe Brook stole Mr. Prince persona from Sanji.
I also think that the character must be know or hinted since part 1. Specially in the last Saga this should be one of the biggest factors to make people accept the new member. As a example, if Bon Clay appeared somehow and joined I think most people would be trilled instead of asking why (and Franky's VA would get extra money). A giant connected to Broggy, Dorry or Saul could also fit the bill.
And last, but not least, Luffy should have asked to join and the person accepts or the person asks to join the crew and is accepted. Basic example: while she wasted pages claiming she wanted to join the crew and stealing the thunder of better characters, when directed asked to join by Luffy Yamato actually denied. Usually Luffy asks at the end of the arc but there's exceptions.
Small detail: I don't believe there's any other character besides Tama that directly asked to join and was accepted by Luffy or that Luffy asked to join and accepted but still is not a permanent crewmember. One more reason why tama=nakama. Frankly, I'm willing to be proven wrong because if this happened it would really be a surprise. And no, Vivi farewell scene is her denying the request to join unfortunately and not like Jinbe.
Putting all this there's very few characters that fit the bill:
>Hancock/S-Snake: Hancock fails because of lack of relationship with the crew and how powerful she is. S-Snake looked like a good compromisse until she was captured in the bubble instead of being left outside to interact with the crew after she freed them. If she was meant to join, the group could have some fun moments with her.
>Lucci: no dream, no interest in interacting with the crew and is actually a antagonist (but without a redeeming flashback or misunderstanding like franky). No reasonable chance of joining.
>Kaku: I mention Kaku just because compared with Lucci he is way more amicable and would have more chances to join (comparable with Lucci). While a cool character he does not have a dream or a good rapport with the crew.
>Bon Clay: stuck in Impel down. However if he somehow appear in the final war he will stay with the crew simple because he's awesome. Sadly this will mean even less Franky scenes but it's a sacrifice that I'm willing to face. However if Flamingo escapes Impel Down for some reason he will not be forgotten and many people will cry manly tears with the reunion.
>Vivi: The issue with Vivi is simple that her position in the world and role in the story does not mix very well with the SH. She works incredible well as a ally but not as a permanent member. And this is even ignoring her lack of dream and role in the ship.
>Tama: There's plenty of reasons why Tama=Nakama, but I already mentioned before in other posts. The only thing to mention is that she was hinted almost 400 chapters before she actually appeared, hinting of her importance in the arc, and actually fitting the bill of a character linked to part 1.
And that's it. Good night folks.