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.
This spell sounds like it might be useful, but I've never been the companion type. I'm okay with summoning some help now and then, but I've always survived by a careful combination of fighting and retreating when necessary. I've never used companions since they tend to die really quickly if you're retreating to save your skin. Although, if you can reanimate many of the things you kill, it might an interesting substitute for summoning.