I posted this in an earlier thread, but it ended up being the last post on a page so it's likely that no-one read it. It was off-topic anyhow, so I'll give it its own topic.
My guess is that the next arc is the next One Piece 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. This Ship's Graveyard or whatever is just the hunting ground for all of the fishmen - they wreck any ships passing through, take all of the loot, and enslave or kill the surviving crew of each ship. Jimbei rules over the island with an iron fist… er... fin-fist. My guess is that Tom and Kokoro left "Paradise" 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 an underwater version of 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 Skypeia. But that's not where the prognostication ends!
Of course, Luffy et al. will somehow avoid capture in the Ship's graveyard and use Franky's expertise to convert the Beast King into a submarine (maybe that's what the soldier dock system is?). Indignant over this fishmen rule thing, the Mugiwaras will start a revolution of sorts. Eventually, they'll have someone strong 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 and takes on Nami?) and bring an end to fishman rule, somehow ushering in an era of cooperation in the Underwater Paradise. Then their new fishperson/merperson/whatever ally joins the mugiwara crew and lives in their aquarium. I mean, come on, why would Oda even mention an aquarium unless it serves a purpose?
Lastly, after all this **** goes down, Dragon magically appears, and comments on how his long-lost son is doing a pretty good job of helping him, even if they technically aren't working together. Mysterious rain and lightning as Dragon gives a badass pose. The Gorosei are now absolutely terrified, as they are down another Shichibukai and also probably think that father Dragon and son Luffy are in cahoots. They might even have to raise some bounties again.
And one more thing, directed to anyone complaining about there not being any need for a new nakama, new nakama don't necessarily need a specific job in order to join. 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.
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-fish or something, heh.