It seems to me that one of the MAJOR complaints folks have about the game is loot balancing between classes. It seems to me that one thing that might make caster loot that much more palatable are scrolls for spells that can not be purchased or taken at level advancement.
Maybe, and this is just a thought, as you work to include new spells make some/many of them like this. So they *only* drop. The Bigby's spells and teh Prismatic spells would be a nice place to start. Mass resistance, Mass hero, etc. Nothing critical or core, but different things.
Just a thought.