I would suspect that Gandalf the Grey would be next.
After that? Well, we know that SS must be working on smaller bodies for this and the SW lines. What better way to introduce it than with a nice Gimli to complete the Three Hunters? He could come with plenty of different axes, and I'll bet they could even figure out a way to give him his exclusive Lothlorien cloak and Galadriel hair gift!
Like most of you, I think it will probably be Gandalf the Grey so that the first 3 figures are 3 of the most recognizable/desireable to the general collecting audience.
After that, I like the idea of the Wiki as the costume would really be able to showcase what Sideshow's craftspeople can do. But I would imagine they'll do a regular Ringwraith before they do the Wiki.
I agree that GTG is the most likely candidate for the 12" but they haven't done a baddie yet and I think we just might see one from that group. Not to mention I think GTG has a good chance at being the next PF.
I'd love to see GTG next. I'd also like a Armored Theoden (w/helm) that would be great too.
As far as a GTG in P.F., I'd probably get it. Of course, it all depends on how good it looks.
I don't plan on going full tilt on the P.F. LOTR stuff, just a few of them, and GTG is one on my list. The 12" stuff, hopefully I will be able to get them all.