Star Wars is and dies with Vader. They know this, so be sure that at least in one shot anakin's ghost or vader's suit will show up, or come back in some form.

I just hope they bring up something from the past that would put Luke in a different quest. Pretty good formula, as seen in ESB. He would have to look for this ancient thing from the old republic in Dagobah, The old Jedi temple, Vader's fortress, etc. And he would find some truth in those places, that could have an impact in his persona (for good or bad) and eventually set up a new mision that configures the general plot of the trilogy. Meanwhile Han and Leia have their politics in the new republic, and their issues with the imperial remnant. We see best from PT and OT.

Pretty much like in The Hobbit, where the bad guys don't have a personal problem with Bilbo, having no antagonist for Luke doesn't mean it won't be a good movie. Have him go down a meditative lone path, if he is the oldie he could make for a Gandalf too.