I'm working on a magic system for the alternate-d20
Universal Decay system. It is not "d20Modern", but conversion should be simple enough.
The idea is that all magic is based on the skillful manipulation of magical energy (Mana) in accordance with practiced areas of endeavor (Spellcasting Feats). The use of these elements is needed to "cast a spell", but there are
no wrote spells like in D&D and d20Modern.
All spells are cast using the Spellcraft skill, and share a few common characteristics (casting time, duration, range, ect). Each Spellcasting feat has a few cahracteristic alterations for spells cast using it, as well as what skill it is linked to and a small list of effects that it can be used to generate (with approriate Mana costs). The saving throw DC of the spell is based directly on its Mana cost and linked skill's ability modifier, with the amount eh save is
failed by determinign how strong the effect is (up to the amount paid for). You can alter a characteristic up or down one or more steps by mdoifying the spell's Mana Cost and Spellcraft skill check apropriately.
The Spellcraft check DC is based flatly on the amount of Mana spent on a spell, but you are limited in how
much Mana you can spend on any spell to an amount equal to that spell's linked skill ranks + linked ability score modifier. Only a few skills are used for linked skills, so there is no need to worry about "skill tree waste". In addition to this, every 5 ranks in a linked skill gives a +2 bonus on Spellcraft checks to cast spells from that skill's Spellcasting feats.
Characters generate an amount of Mana every day equal to their Character Level when they wake up, and can contain an amount equal to their CL x Spellcraft ranks. Rituals and talismans can also be used to accelerate Mana regeneration or gain free Mana from the area to offset the costs of spells (this is why material components are so often used: free Mana!)
This allows for a very different style of spellcaster than typical d20 games, since all a character needs to effectively get an entire "chain" of D&D-style spells (and several related spells) is one feat. Characters do not rely on wrote memorization for spells since they are all now effectively "spontaneous casters", but having a particular spell written down for reference is STILL handy since it provides a Spellcraft bonus when using it during a casting
So far I have had a very posative result using this system.