I started reading OP three years ago or so and have been a lurker on these forums since near the beginning of the Water 7 arc, but now I'm going to take a dive and post some ideas.
First of all, a mermaid/fishman/whatever doesn't necessarily need to fit in the crew. Vivi didn't have any particular role in the crew, and Robin's role in the crew is completely unspecialized if there aren't any Poneglyphs around. Not every nakama needs a job; the only real qualification is believing in Luffy.
My guess is that the next arc is going to be a very relatively short one, about getting through the ship's graveyard. Then we get the next Odacchi epic - the Underwater Paradise arc. My theory is that Underwater Paradise is an island populated by both merpeople (like Kokoro) and fishmen (like Tom and Arlong). The fishmen rule over the merpeople with their insane strength, and years of said oppression has given all fishmen a nasty superiority complex. Jimbei rules over the island with an iron fist… er... fin-fist. My guess is that Tom and Kokoro left in the first place because Tom wasn't a fan of the situation and took Kokoro with him; she might have even been his lover or something.
Now that sounds a lot like Skypeia, doesn't it? I'm sure if my theory turns out to be true at all, Oda's going to have a lot of twists and turns and thematic elements that make it much different from Skpeia. But that's not where the prognostication ends!
Of course, Luffy et al. will be indignant over this fishmen rule thing, and will start raising a ruckus. Eventually, they'll have someone on the inside helping them out - this could be a mermaid, a fishman, or whatever the hell else Oda dreams up. I'm also thinking that it could be a human with a fish-fish fruit of sorts that has been masquerading as a fishman for awhile (using the in-between form). Of course, this totally retcons the idea that all DF users are hated by the sea, as a fish-human would be kind of pointless if that rule was absolute.
But anyway, back to the conflict. Jimbei is from the same school of thought as his old nakama Arlong, so there's going to be a lot of derision of the Mugiwara crew as this arc moves on. With the help of this new ally, Luffy and co. take out Jimbei and a bunch of other hella-strong fishmen (maybe Arlong comes back much stronger and has to take on Zoro with a new Kiribachi?) and bring an end to fishman rule, somehow ushering in an era of cooperation in the Underwater Paradise.
So yeah, that's my theory early on. I am probably completely wrong, of course, as Oda has surprised me a whole lot before in the past (totally didn't see it coming when CP9's true identities were revealed, for one), but right now I'm thinking we might get something along those lines. Right now I just want to see Usopp return to the crew (even if it's as Sogeking) and for Zoro to get a new sword. Who knows? Maybe Tom's estranged brother is a master swordsmith or something, heh.