I see the next story arc being Loguetown-esque. The whole Water 7/Enies Lobby saga has really given me Arlong Park vibes (crewmember's departure, new bounties, heavy emotional tones), so I think next we'll have some check-ups on some long-running characters, mostly showing reactions to the events of Enies Lobby and also reactions to the new bounties of the Straw Hats. Like Loguetown, I think the next arc will be short and for the most part light-hearted, but then cram in some major plot development in a small space. Then, to mirror Loguetown even more, Blackbeard and his crew, or maybe even Smoker's gang, will finally catch up to the Straw Hats and they'll have to flee for the island. Then I think they'll cross the Red Line and enter the second half of the Grand Line, in a similar way that they entered the first half.
However, I could see there being a few things before the crossing of the Red Line. The Straw Hats could go to Fishman Island first (and presumably face Jinbei), or Doflamingo and Bartholomew Bear could show up as the next major villains. I have a feeling, though, that the next major arc won't be as long as the whole Water 7/Enies Lobby arc, and I think it's about time that Luffy took down another Shichibukai (Jinbei or Doflamingo are my guesses for the next Shichibukai he faces).