The monster trio is not a trio long ago. Oda is constantly making clear now it's only a duo. It's Luffy and Zoro. Sad, but he is not treating Sanji as the third powerhouse for years already.
But, well, Jinbei for SH… I will simply quote my opinion from other post of mine (but now in the right thread):
! @.access:
! > If we were close to the end of the manga I would think differently. But Oda already said we'll have like 10 years more (and since he always think things will be faster than they really are, it will probably be 15).
Now… think. The SH are still in the middle of their adventure, do you REALLY think they will get a member who is part of one of the strongest groups in the world?
It's like getting a Lv.85 character for your party in a RPG game when the game is still around level 45~50.
Like if Rune from Phantasy Star IV or the glasses guy from Tales of the Abyss stayed in the party with their regular levels or something like that...
What would Jinbei do in the SH in this mean time? Sit on his ass? Because there is no way any "minor villain" would stand a chance against him. Or would he be fighting the major villains instead of Luffy?
That or he would have 10 years of easy fights... or Oda will have to start pulling out Shichibukai level people out of his ass in every crew or organization the SH fight against (one for Jinbei, one stronger for Luffy - since he is the captain and only god knows how Luffy would manage to get stronger than Jinbei so fast -, another one for Zoro not look weak in comparison...).
Jinbei is almost at the top of the world. Along with Yonkou and the Marine, there are the Shichibukai. Jinbei will lose his place in the group, but he is still Shichibukai level... how a fully developed fighter will be able to join a group that is just starting to mature (more: exactly in the moment of the story they realized they need to grow a lot yet. I.e. when their current level is at the center of the stage)?
Well... but this is still OP and Oda. You never know, maybe he joins. If he does, Oda will have to out do himself in order to manage Jinbei in that group for at least 10 years (or make Jinbei get considerably weaker and start to regain his powers little by little over time...).
Not to mention I simply can't imagine his personality fitting the excentric Straw Hats. He looks… too "normal" compared to them.
About Margaret... again? I thought this idea died already. She was left behind and Luffy showed as much interest in her as... Conis (equal zero).
My favorite for joining the crew, for years already, was Buggy. I got pretty excited when he came back... but seeing how much Oda is making he sound like Usopp it's kinda impossible to have both in the same group.
And his newfound partnership with Mr.3 kinda settles it... sadly :/
Mr.2... it could really be, but I don't know. At first surely looked like Oda was trying to make Bentham a 'SH material'... but it looks like he completely lost his interest now. It's like he simply made Bentham shine soon so he could work the other characters.
I believe it is more likely he will join the revolutionaries together with Iva-san.
The other ones in the team I don't even consider the posibilities (Inazuma could have some if Oda decides to develop him... unlikely, though :/)