As of now, the lead speculation is that it will be a Sith Eyes Anakin. From all the hints, and the fact that usually the SDCC SSC Exclusive is some kind of retool/variant of an existing figure, that's my best guess. It would make the most sense, since it would be a simple eye repaint, some tattering/burn damage on the clothing, and knowing they have SDCC coming up produce a slight retool of the existing head cast to give him more of a "messed up" look. Remember the old adage "Why make one, when you can make two for the price of one".
If memory serves me right, I believe Dave Myatt said somewhere that it won't be a Han or Luke, or any figure in current 1/4 format. Take what you will from it, but here is a little interesting tidbit: (sixth question from the bottom)Interview with Tom Gilliland