I don't know really.
But, in reply to some of the things said in this thread... Though I made this account a while ago and my computer's so slow it took forever to edit my account and I forgot what I wanted to say...
Uh... Jinbe having haki. Yes, it's true that it's never been stated that Jinbe has haki but it's reasonable to assume Jinbe has haki because he was able to fend off lava with his hands. C'mon, your not gonna tell me he has lava resistant skin are you? Though that hasn't been stated either... it's less reasonable. But that's just my two cents. There was some other stuff, forgot it now but... as to who I think the next crew member would be...
First let me say that I remember seeing somewhere a long time ago something about Oda stating in one of his things where he does those... things... where he like... states stuff... anyways... that the next crew member would be someone who is a past criminal/enemy (so someone we already knew before the time skip or maybe before the Grand line, can't remember when I heard that stated so...) and someone funny.
Only people that come to mind for that are
Buggy, but I doubt him, but who knows, he helped Luffy in the arc with Whitebeard and he has to show up again sometime right? Who knows... Maybe Luffy won't get any new member in Fishman Island? Though I think that the next crew member is most likely going to be someone suited for water environments.
Jinbe. He became a criminal by helping luffy and stuff. And although I didn't think him at first because he seemed so serious all the time, some chapter ago when Luffy arrived on Fishman Island I could see him showing signs of possible humor and sillyness along the lines of Luffy, Usopp, and Chopper. But I'm not sure because besides Jinbe who is there to protect Fishman Island? Obviously King Neptune's not enough if Whitebeard helped them before. But maybe something will happen, or maybe Luffy will claim territory of Fishman Island that would solve everything I guess, and allow Jinbe to join Luffy, and help out the Fishman/Human relationship thing.
My last speculation is that guy we saw that had that mud fruit or was it swamp... I can't remember but I don't even know where he's at anymore... lol... but I think their last crew member is most definitely going to be a fishman/mermaid, it just seems so likely at the moment... but I highly highly doubt Shirahoshi will join. I mean what can she do besides the Sea King thing (Which also is just kind of... I don't know, maybe not THAT useful, and can't even be used in some places.) also, she's the freakin' princess. I just don't see it happening.
My prediction right now?
I guess I see Jinbe joining luffy's crew as higher than a 50% chance, so yeah, I'm leaning more towards that side. Also, I don't think Jinbe is stronger than Luffy anymore. Probably not stronger than Zoro either. Unless Jinbe trained those 2 years.
I've forgotten a lot of things of One Piece and other manga I read. Dx I read OP every week though... lol.
Anyways, I just made this account so... what's up people?