Way back when I was talking about tweaking Magic-Users to create variant classes, I mentioned replacing the spell prep and spell casting rules. But I really only gave two examples of alternate casting methods: using scrolls/potions instead of incantations, and taking a penalty to cast illusions without words, gestures, or eye contact.
There are other possibilities that I've mentioned before and that also show up in a lot of fantasy tales and films: spell casting that is exhausting or that causes aging. We can use the same mechanic, with modification, for both.
Exhausting Magic: Roll 1d6 per spell level. If a spell causes damage, you can use the damage roll instead (if it's the same number of dice.) If one of the dice rolled is 5+, the caster is now tired (1 point penalty to all rolls until the caster rests.) If the caster is already tired, the next step is very tired (2 point penalty,) followed by exhausted (collapses, unable to move.) Modify the 5+ target number by the caster's Con bonus/penalty, and by the tired/very tired penalty.
Aging Magic: Same basic die roll, but on a 5+, the caster adds the total of all the dice to effective age (in years.) Use the simple aging rules to make this scary. If a caster is young/old, add an extra die, or two dice if very young/old, three if extremely young/old. You can seemingly cast unlimited fireballs at 5th level, but you'll age fast. Elven spellcasters should get the full effect, even if their normal aging is very slow.