An idea struck me on the head today. What if when summoning deadra or raising dead, instead of paying a flat rate of magika and having the creature for a set amout of time, it depended on the size of you're magika pool. This way the more powerful of a mage you are the longer you can keep a deadra in our realm, or a corpse animated. I'm sure this will come with it's own issues but hit me with them and I'll think of a way to make it work.

I like it the way it is. Also, isn't there a perk that keeps summoned/raised being longer already?