In the fantasy adventuring party, some of them fight and others have spells.
In the superhero party, everyone fights and everyone has one "spell."
(And Doctor Strange.)
|Superknight by John Staub.|
This suggests a "medieval mutants" campaign using the standard rules of your D&D-oid system, but there are no clerics or wizards. Instead, everyone is a fighter or thief (or a scholar, with stats as a wizard and gaining experience twice as quickly).
All PCs and major villains get a mutant power that is a (d6: 1-3, cleric, 4-6, wizard) spell, level determined by the minimum of 2d6. You get an additional, thematically related spell at every odd level. If the power's level is:
Greater than twice your level: Using it knocks you out for 1 hour.
Greater than your level: You can only use it once a day.
Equal to your level: You can use it twice per day.
Lower than your level: You can use it three times a day
Lower than half your level: You can use it at will.
Also, this looks good for a psionics system in a standard fantasy campaign, with an appropriate XP tax on the lucky psionicist.