a necromancer could probably do a ritual on his corpse so he can become a http://www.youtube.com/watch?v=s38TrVza3rU in skyrim
That video gives me hope for the next game. I think Beth has learned what awesome looks like from the community. We'll see...
I would like to see Easter Eggs characters like M'aiq the Liar show up again, but also continue with powerful artifacts like Umbra.
Also, this.
I would bet my last sweetroll that vampires will be in the game. Haven't they been playable in (main) games more often than not? Werewolves are not out of the question, and werebears could show up too. I'm pretty sure wereboars live in another province, but I am hopeful anyway.
Hircine would fit in nicely. Jyggalag has probably gathered himself by the time the game comes out, so seeing the Tin Man again is entirely possible.
Lastly, no ghosts, Probably Bloodmoon creatures, and the Dark Brotherhood was entirely too popular not to show up again. And of course there will be dragons.