It's a tough thing to guess... I don't think they're going to do another prequel figure after two in a row... ANH Obi Wan would be too soon to the PF... I think they'll start a new line... hell there's too many characters they could do, and I can't see anyone as being an obvious next figure except Padme, and she won't be till down the line probably(when they do the EU figure).
So, either Sith(which would be prequel again - but a Dooku would go perfectly with Obi-Wan and Anakin), Military(seems a bit soon), Scum and Villainy(Boba Fett seems too soon without Han, though I guess if they did a ROTJ Boba Fett that'd go perfectly with Luke), or that other line I forget(legendary characters or something like that).
Just an outright random guess, then: Boba Fett from ROTJ.