spells known and spells per day are what can be used safely but not the only limitation
spontaneous casters can use higher level spells or extra spells per day by making a fortitude save of 10 plus the spell level or be exhausted and take 1d4 per spell level non lethal damage. the idea is that since they pull magic freely from things around them that they can push themselves a little further but at serious personal risk
memorised casters since know spells by memory can store there spells per day safely but the spells themselves want to get out they are forcing the magic they learned under control and therefore can memorize higher level or more spells by making a concentration check once an hour of 10 plus the spell level every hour of game play the check dc raises by 1 much like saves for drowning. if they fail their concentration check the spell immediately goes off and its effect is up to the dm's discretion (I'm thinking maybe doing a percentage scale on whether the spell helps ,harms or simply dissipates with no effect.
the idea is to make magic more of a dangerous force make the game a bit more cinematic and allow the pc's to gamble a little more with higher power and still keep it balanced.
I would like to try to come up with something similar for the other classes too so the characters can push themselves a little further ,give it there all to the very limit without using action points. which by the way I'm a fan of . I just want to do something a little grittier