After the Impel Down arc, I say Jinbei or Donflamingo.
And what do you think about the theory according to which Impel Down IS the mermen island ? Remember, Oda said that the mermen island is just half-way to the end of Grandline. And Impel Down is said to be near Mari-Joa which seems to be, and many details tend to prove it, on the second connection between Grandline and Redline, in other words, half-way to the end of Grandline…
Impel Down, some kind of SUBMARINE prison, does it ring a bell ? But I don't think Jinbei or even Blackbeard deserve an entire 10 vol.-long arc only for them. It may be time for Luffy, as he is quite the big rebel type now, to encounter Dragon.
I like the theory according to which the last three big arcs were element-based (Alabasta-Fire, Skypiea-Air, Water7 - Water) and that we're going to get an "Earth-based" arc next to this one...Why not a Redline crossing ?
I also like the theory according to which, FF and the Galerians are going to be jailed in Impel Down, and the WG will use it as a pressure on Luffy to make him deal with Doflamingo. They understood he's a threat to them, but they can't take him out publicly without distrusting their own system...So the send Luffy to do the dirty job for them...
All these ideas sound smart to me, but I know Oda will go for another, totally surprising way