I dont know if anyone has talked about this yet but I have heard a rumor about a spell called Reanimation. This spell allows you to bring back an enemy that you just killed and it will become your minion of destruction. I think that this spell would be cool. Go out and kill a daedric god and bring it back to life and let it reak havock along side you. let me know wat you think.
I didn't mean to imply that summoning was out, just that reanimation would be an interesting additional choice if treated like summoning (ie disposable help). I actually don't know whether summoning will be in or not. I wonder how well you could control a summoned Daedra while battling the forces of Oblivion...