very long
Heh. I've been wrestling with this question for about five years, now. While I was trying to adapt my world to the Aria system, I started asking myself some tough questions. I went into it with three basis tenets. 1) In broad strokes, most real-world magical beliefs should be available and systemized. 2) No "generic" system like Mage: the Ascension -- each system should be fleshed out. 3) Magic is extremely potent, but it comes at a price.
What I came up with was 5 arcane "schools": Thaumaturgy, Runecasting, Witchlore, Artifacing, and Necromancy. Add in essentially two modes of divine power: shamanism/infernalism and true priesthood. Top it all off with some misfits: Psionics, Sorcery, and Geomancy. Oh, yeah, did I mention that there were some cross-over abilities? It all came out to a fairly interesting system (my players were psyched), but difficult to translate to d20.
Thaumaturgy basically comes from using words to directly and immediately control the fabric of reality. Basically, every effect would have a duration of "instantaneous". This is the school that casts fireballs, magic missles, wish, etc. Some spells could be maintained so long as the wizard continued to chant and keep his control over reality. Since thaumaturges handle raw power on a regular basis, they risk mishandling it and causing horrid mishaps, injuring or even killing themselves.
Runecasters use mystic symbols and rites to unlock the power within inanimate objects. They specialize in protection and scying spells, although they are no slouches at healing (usually in the form of regeneration). Runic spells feed from the object they are inscribed upon, ultimately destroying their host object. Effects last as long as the rune does, whether the symbol is marred or the object is consumed by the spell. Runecasters often scribe runes upon their own bodies, or upon others. The ability of a living being to heal itself usually washes with the consumption from the runes, but extremely potent symbols are not advised. Ultimately, many runecasters are driven insane by the consumpive effects of their mysteries.
Witchlore is to life what runelore is to the inanimate. A witch is expert at unleashing the energy within living things. Most of their spells are delivered through a potion or salve. Still, none match the ritual power of a witch. With enough preparation, a coven of witches could bestow fertility upon the fields for a dozen or more miles, or they could curse them with barrenness. Depending upon the strength of the effect, a witch needs progressive stronger sources of fresh (or properly preserved) living matter. A bit of herb and water is all that is required for a healing potion, while the aforementioned curse -- or blessing -- of the land would require the sacrifice of a sentient creature. Because of their link with life, many young witches are stunningly handsome or beautiful. Often, however, an elder or daringly power-hungry witch is horribly disfigured by feedback from her spells.
Artifacing is rarely learned on its own. Rather it usually augments one of the other arts, instilling their power into an object on a permanent basis. Artifacing relies on arcane techniques to tap into free energy rather than feeding upon the object itself (as runecasting does). By itself, artifacing using results in making an object more ideal. Thus, a sword becomes more sword-like, striking better and leaving deeper wounds. Armor would, likewise, become more maneuverable and more protective. Because of the focus on doing things slowly, carefully, and correctly, artifacing is probably the safest school and has no noteworthy effects -- other than the occasional dedicated artifacer dying of boredom.
Necromancy is where things get interesting. The spells themselves are probably the most versatile of the arcane spells, with almost any effect possible and no unwieldy instruments like the witch's brews or the runic stones. Necromantic arts are also among the easiest to learn and have great inherent power. Every sentient being hates the necromancer, though, because he draws his power by the consumption of souls. Although a necromancer need not kill his victim to harvest his energy, the damage done by a necromancer will _never_ heal, even in the afterlife. The effects on a living subject is usually a deadening of emotions, resulting in a depression of sorts. Because this deadening is tied to the soul, it will never heal. Anyone completely drained by a necromancy for power is gone forever, even beyond the reach of the gods. Although elves have stronger souls than those of most races, they have an even greater loathing for Necromancers -- where most beings are beyond the reach of a necromancer after they die a normal death, the elves are eternally reincarnated, carrying their spiritual wounds with them into harm's way once more. Aside from being a pariah among pariahs, the necromancer faces the possibility that he will become so desparate for power that he taps into his own soul for power in a crisis. Of course, that may be preferable to what awaits him in the afterlife as the lords of the underworld extract payment for any souls he may have destroyed before the fiends could claim them.
Although most wizards learn only one art to any degree, any can be mixed to create effects with properties of each. Most common is mixing artifacing with any of the others, with obvious results. Only slightly less common is mixing thaumaturgy and runecasting, allowing the wizard to create runes that can trigger complex effects or to trace patterns into the air, creating a conduit of power to extend thaumaturgical spells.
Divine magic is essentially broken into two groups: those who call upon multiple entities, depending on what seems most reasonable at the time; and those who devote their entire lives to a single master. Regardless, the price and the power are much more explicit in this relationship. Whether it comes in goods (offerrings) or services (prosetylizing, acts, etc.), each priest much gain favor in the eyes of whomever he wishes to call upon. The price is devotion, but the benefit is the ability to call upon the aid of a being(s) that put the most potent wizard to shame.
Psionics is similar to thamaturgy in that it taps into the fabric of reality to set effects into motion. It differs in that, where anyone could potentially learn thaumaturgic (or any arcane) rites, psions are born with inherent power. They work their magic by "priming the pump" with a bit of their own power and channelling raw power through themselves. Because of the flood they unlease, they may burn themselves out, scarring whatever mechanism allows them to channel.
Sorcery is an art reserved for the trollish race. These are not the trolls from the Monster Manual. Rather, they are highly intelligent giants that have a strong tie to the plane of shadow. Some members of the race learn to manipulate this link. They discover how to pull bits of shadow into the Prime. Sometimes they forge shadowy weapons with which to fight. Sometimes they learn to emulate arcane magical effects with their shadows. The nature of troll sorcery is poorly understood by non-trolls. This is not surprising, since the trolls have only been about for roughly 250 years. It is assumed that more insight will be available when their origin is known.
Geomancers are the least understood of any of the spellcasters. It is entirely possible that they do not even understand it themselves. Geomancers work their effects through gemstones -- they place the stones in certain ways, mount them on amulets, in weapons, etc. It is whispered that geomancers tap into a power more primal than the gods and spirits; more primal even than the arcane weave. Whatever it is, this power that lurks in gemstones appears to be sentient on some level. Some geomancers have commented that they are not in control of their own fates. Certainly, the more potent of the gem mages seem to be surrounded by an endless stream of unlikely coincidences and seem to have a strange faith in their own destinies.