Voice Acting is the only thing you can recycle from game to game without having much trouble to adapt it into the engine.
Besides I really would like to see some of the creatures of Morrowind/Oblivion integrated into Skyrim (if they fit in lorewise, like a boar), but this is on the other hand more difficult.

Anyways if Bethesda won't do it I really hope that there will be a mod for this.