I had something very similar to a setup arscott had for my own home brew.
The multiverse is suffused with energies far too potent for mortals to handle. In the distant past mortals once knew how but the results were far too drastic. The immortals are unable to control the energies themselves, so they instead created a barrier preventing mortals from drawing upon the energies directly.
The immortals themselves allowed their own followers to draw upon their own limitless divine essence to power magic. The immortals knew that due to their own limits, they couldn't prevent mortals from finding ways to bypass the barrier. So they then began instructing their followers on how mortals could tap through the limiting barrier and not have total access to unlimited power. As long as mortals would draw energy through the barrier, the immortals believe there is little danger mortals could once again threaten the balance of the multiverse. To bypass the barrier, immortals taught mortals on drawing energy through living things. All living things by their nature have a supernatural connection to realm of the divine.
(Note: this world is essentially my own creation of the world that precedes what Dark Sun becomes. I have my own pre-history development that ditches the published history and timeline. So, this effectively describes "preserving". Mortals haven't learned "defiling", yet . . ..)
In this way, there are some schools of "Spiritual" mages. Each is an arcane class with its own finite spell list. Each list of spells tailored narrowly along certain interests of important immortals. Available to PCs are the "spiritual" arcane spell lists of Nature (mostly animal, wood, and earth), Savant (mostly divinations, law, and force), Sea (mostly water, sea animals, and wind), Song (mostly sonic, charm, and conjuring), and Storm (mostly electrical, weather, and chaos). Some intrepid mortals are secretly experimenting with new ways of combining "scientific" principles with the newly ways of drawing energy. Thus there are the "science" spell lists of Alchemy (pretty self explanatory), Geometry (symbols, line, runes, numerology), and Mindbending (the seer and telepath psion class). There are some other prestige classes exclusive to casters who chose a "science" spell list.
Needless to say, there have been some malign entities who are trying to subvert the Immortal's barrier and restore their previous unimpeded connection to mortal spellcasters. These are the "Lost Paths" of arcane magic. Many of these schools require the mortal to make a pact with a supernatural entity and then get access to a single spell list. Each Path is its own spell list. There are Paths of Elemental Air, Elemental Earth, Elemental Fire, and Elemental Water, each of these Paths require a pact with an elemental lord. There is the Lost Path of Hedge Magic, the spellcaster makes a pact with a fey spirit. The Lost Path of Fiendseeking, spellcasters are called warlocks (with the new warlock class, this is made easy for me) who make a pact with evil outsiders, demon or devil. There is the Lost Path of the Deathmaster, a necromancer who has a pact with an ancient incorporeal undead entity. There is the Lost Path of Ever Cold, a spellcaster who has a pact with an supremely powerful mortal entity who couldn't be killed, so he who was imprisoned in a cold dimension. Cold mages have learned to pierce the dimensional prison enough to establish a psychic connection to the entity. There is the Lost Path of Shadow, spellcasters who have made a pact to sacrifice portions of their material form to a shadow entity so as to draw energy from Shadow.
Needless to say, every arcane spellcaster has a dedicated spell list. Each spellcaster draws energy from specific and defined energy sources. With the plethora of d20 sources out there, it has been very easy to recreate these old 2nd ed. inventions of mine as 3e versions.
I have a future-history planned out that will take this system towards Dark Sun's status quo tens of thousands of game years later.
Regards,
Eric Anondson