Why not try the "Spell Law" book from Rolemaster 2nd edition, as it can be slotted into most of the standard system types, including d20. The magic system it introduces is spell-point based, and spells are learned in a list form. Each list is a collection of similar spells, but of differing applications. It also does away with the D&D 0-9 spell levels, as spells go up to 50th level. It also has some very good mechanics for governing spellcasters who cast above their level, which is entirely possible.
There are tons of spell lists and spells, but as a GM, you would need to assign different lists to different classes.
You can find a PDF copy online quite easily, if you wanted to have a look.
