All your base are belong to us.
I hit on this simple expedient. The minimum INT required to learn a spell was the spell's level+11; e.g., a 4th level spell requires an INT 15 to learn. Each spell was treated as a separate skill. The spell-caster's "level" for determining the power of the spell was the skill divided by 5. The Magic Point cost for each spell was equal to the spell level, plus additional Magic Points for extra range, duration, etc. For example, a Wizard with an INT of 13 (or more) knows the second level spell Detect Invisible at 45%. He could cast it as the equivalent of a 9th level D&D wizard with a range of 90', but it would cost him 10 Magic Points (2MP base+8 for extra "levels" of effect). However, a magic user can opt to cast a spell at a lower "level" by pumping fewer Magic Points into it. So our wizard could opt to put only 3MP into the spell for 20' range, etc. A Critical Success means the spell only burns the minimum number of Magic Points but takes effect at maximum level (or less at caster's choice). For example, our wizard knows Fireball at 45% and gets a Critical Success. The spell only consumes 3 Magic Points but does a full 9d6 damage. A Fumble means the spell is a dud. It uses up all the MP put into it but has no effect.
You don't say.
That's all there was to it. Nothing too earthshaking, I admit. It worked OK, although I didn't playtest it extensively. It was only used in a few encounters with NPC's on alternative worlds. Other people have thought of their own versions of this long before I did. I saw and old Pegasus magazine article where someone had come up with a very similar system, although he also added (IIRC) a pool of points called Mana that limited the amount of magic that could be cast in a certain time period. And I can think of plenty of variations. You could have a character pick a class and that class would have a % ranking, starting at 5% ("first level"). This would function as the Level your character has in their "class". So a Druid 30% would be the equivalent of a 6th Level D&D character. Etc. So don't just limit yourself to plundering dungeons. Plunder those other games for rules, monsters and magic, too.