If this is in the wrong place, please tell me and I'll gladly gtfo.
I'm sure this has probably been up a few times, but I can't find it (or rather, I'm too lazy to), but I just wanna get my POV out there and get some other fan's ideas too. Also, I'm an obnoxious little shit, so you'll have to excuse the excessive nerd rage here.
Okay, so a lot of people are saying that Boa Hancock will be next to join the crew- I'm not sure why so many people believe this, though methinks it has something to do with her gigantic tits and her devil fruit ability of MAKE YOU HORNY. (In case you didn't notice, I'm not a huge fan of Hancock). It kind of annoys me that one of the first female powerhouses is just some sex object, with a personality so annoying it makes you want to rip your testicles out), but weird masculine feminism aside, she's blatantly -not- a Straw Hat Pirate. Why, you ask, Internet? Because she's a captain of her own crew and abandoning them would make her pretty shit (or moreso). Also, she's ridiculously overrated, despite her crappy devil fruit ability -being the Empress of Amazon Lily, captain of the Kuja Pirates, Shichibukai, most beautiful woman in the world, overall, God - to leave her crew behind and jump ship with Luffy just because she wants to hump him. It doesn't mesh, does it?
I personally think that Jinbei would be a much better candidate because he fulfils the role of 'fishman' that Luffy wanted. He's got some pretty awesome abilities but he's not a devil fruit user, which is cool because I don't personally believe that the Straw Hats need another one of those. I also thought that, even though he told Ace that he only protects people he cares about, he seems to be protecting Luffy anyway like Ace wanted him to- he took him back to Amazon Lily and stuff.
I guess the other characters who I think might come into it are Keimi and obviously Pappug, because she's a female character and she would also fulfil the fishman/woman role, or Mr 2 because honestly I, like a lot of people, don't honestly believe that he's dead, because you didn't explicitly see him die and I don't think Oda would've given him such a death.
Another reason I believe that Jimbei is going to be the next crewmember is because of Blackbeard's crew. Oda nearly always designs enemy crews to correlate with the Straw Hats, so that each crewmember has an opponent they're well matched with. So, this is what I think:
Blackbeard vs. Luffy (pretty obvious)
Van Auger vs Usopp (they're both marksmen)
Jesus Burgess vs Zoro (wrestler, very strong)*
Doc Q & Stronger vs Chopper (doctors, animals etc)
Lafitte vs Sanji (tap dancer, which suggests he uses his feet for battle somehow, but obviously uses hypnotism and apparently can transform his arms into wings)
Shiryuu vs Robin (just a hunch, really, also because he was a prison guard and whatever, and Robin has been wanted since she was a kid, I don't know)*
San Juan Wolf vs Franky (there's a lot of references to Franky's appearance in his, such as the thick black 'eyelash' lines beneath each eye, so maybe its a forerunner or something)
Basco Shot & Abalo Pizarro - these I'm not sure about, I think Brook will end up fighting one of them and perhaps whoever joins the crew next will fight one of them, so I'm not all that sure about these.
Catarina Devon vs Nami, just because Nami nearly always has to fight the female enemies, like she did with Miss Double Finger and Kalifa.
*I think that Zoro will also fight Shiryuu because he's a swordsman, so maybe he'll fight them both and Robin will go against Burgess or something. I don't know, but either way I can't wait for those fights.