I'd think there's not going to be any more, personally. And even if there is, at BEST it'll be like one. I heard somewhere that the crew was going to be around ten, IIRC... I believe it was Luffy who said it come to think of it... Even then however I can't honestly think of what's still needed. I mean seriously, they have everything that's needed of a crew (Each with uber abilties in given field), and stuff that's really unnecessary. Like an Archaeologist. Hell, the run on joke was that every time Luffy needed a new crewmember of a specific type, he'd say they needed a musician first, and now they've got that one. Personally I think they're done.
Well, save Vivi, but at this point it's impossible until they get to the end of the grand line and loop around again. Unless we have important POLITICS somewhere in the new world that requires her to be there, but even that would have to be REALLY late in, given she's still there from that chapter not too long ago, and the Strawhats have made it halfway around the world already. Unless Pell is more uber than we're led to believe, and even then I'd only buy a couple of months of travel. We are talking around the world afterall.
However if I'm really reaching, I'd say Hancock. For epic lulz.
I suppose if there's really another job to be filled, they MIGHT get something along the lines of a mad scientist inventor or something... but Franky, Chopper, and especially Usopp have that one covered.