What I'm dreading is a "new" villain who is actually a young clone of Palpatine. Worse, he's not even "clone", he is created by Palpatine only through the force (to bridge with Ep. III").
Oh, the horror...
If there's anything I'd like to see from the Prequels, it would be the Separatist capial ships from Ep. III. Fantastic designs which were greatly underused.