I still think Ben should play Luke, he's good at being the hero. Obi-Wan is his character, but he would work as an NPC. Leia makes sense as an NPC, too, she really doesn't do that much. Han really has to be Jim, nobody fits that personality more than he does. Pete of course is R2, and I think Sally will start as 3PO but switch to Chewbacca. This would leave Annie as the GM, which makes sense, Vader is her character, plus the Han/Leia romance would be awkward if it was handled by Jim and the current GM.
But where would that leave our current GM? Well, maybe he'll take a much-needed break and come back in the second movie as Lando (with possibly a cameo appearance as Biggs and/or Wedge). Him and Annie being in cahoots in the Cloud City parts makes sense to me.