So I've been mulling over magic a bit. I think I have a useful mechanic, but let me know what you think. For starters, let's say you stick with the current spell slots per day system in wizard, cleric and sorceror models, since it's easier to port things that way. So then I made up a feat, Channel Spell, which basically allows you to cast a couple of extra spells each day with a little risk. Instead of using up a prepared spell, or a spell slot, you can attempt to just cast a spell out of nowhere. It has to be a spell you either had prepared that day or know how to cast in the case of spontaneous casters. Make a spellcraft check DC 15 (?) + spell level, success means you cast the spell. Whether you fail or succeed, you are knocked one step down the condition track from exhaustion. If you try to use this feat again on the same day, the DC increases by 5 each time. This way you're going to get 1-2 more useful slots, that scale with your level, skill focus will give you another and further feats/talents could be added to the system to say, avoid the condition knock if you succeed, etc.