All spell casters gain their abilities through a form of demon possession. So in order to gain magical abilites, PCs must track down and trap demons. Of course there are some pretty powerful demons, if the demon is stronger than the PC, the PC could find themselves subjugated to the Demon's will.
In your world the pantheon is a rotating system of formerly human individuals. You have been selected for god-duty for the next 2,000 years. How are you going to get out of this?