gamerprinter
Mapper/Publisher
Drain Magic
When a caster casts a spell the caster takes damage. You can cast spells till you die, thats the only limiting factor. (the potential problem I see with this in D&D is healing magic as they dont have a drain stat)
Improv Magic
You make up your spells on the fly. There is no spell list, sometimes it may have a component or gesture requirement depending on the fiction. Most games like this have a couple tables (area, power, etc) and a skill roll to see if you succeeded in casting whatever you wanted. For example in Mage I could be running away from some goons and decided I want to create a banana peel under them or that I want to teleport to Mt Everest.
Although I'm not as adept at Psionics rules, one of my players played a psionicist that had an ability to drain/take ability damage in exchange for enchancing other abilities or otherwise enhancing a powerful psi attack.
Perhaps take deliberate CON damage per spell cast, limiting your spell capability to your CON score, zero CON is death. Ensure to have some restoration spells handy to keep on casting a bit longer than your stat score. If you should be able to cast more spells, make it two spells for one CON, or whatever other measure works best.
My previous post pretty much handled my way for your improv caster. If I could recognize when a casters impromtu spell as equaling a 5th level Wizard's spell, then I assign the spell to the slot and expend it. Continue until all your slots are gone.
Know this, that I'm a startup 3pp, and I've been working on settings for Pathfinder, that have rules specific to the settings, but are designed to integrate well to PF Core. So this kind of exercise is what I've been doing for much of the last year or so in RL.
GP