The Tome of Battle: Book of Nine Swords is growing on me. At first, I sort of thought it was an Exalted clone in D&D clothing. My first impression wasn't wholly misplaced, but I think that the primary inspiration was a bit closer to home. Ultimately, my take on Bo9S is that it is an alternative magic system for D&D. In many ways, playing a Bo9S character is more like playing a core D&D spellcaster than a core D&D fighter. The maneuvers are a lot like spells. Not only are they organized into nine levels, but they have durations, targets, 'casting times,' and all the other trappings of the d20 spell system. The maneuver system and the spell system are rather similar.
The big difference? The way you learn, prepare, and regain the ability to use maneuvers is different than spells... and, in my mind, superior. The 'daily use' of spells has always bugged me. It would be rather easy, though, to plug spells in for maneuvers and get a Bo9S-style magic system.
Each of the three classes in Bo9S each treat preparation and recovery of maneuvers differently, and these could easily be adapted to different magical styles.
The swordsage knows a ton of maneuvers. It takes the swordsage a substantial amount of time, however, to change the maneuvers she has prepared or recover the ones she's used. This is analagous to a wizard. The wizard under this system could stop and read her spellbook to change prepared spells or re-prepare spells.
The warblade knows few maneuvers, but can use them near-constantly. This is analogous to a sorcerer or warlock. Such a character might need to spend a single round concentrating or somesuch to recover their magical energy in full.
The crusader knows several maneuvers, but doesn't have full control over which are available at any given time. This is actually a pretty cool style in play, since it encourages creativity. It would match up well to a wild mage sort of character.
I'm also a fan of how maneuvers are learned. Some high level maneuvers have prerequisites - they might be other specific maneuvers, or they might be things like 'two other maneuvers of this school.' This makes sense for spells as well.
'Caster level' under this system would also benefit. Bo9S characters have an effective 'caster level' equal to their level in the martial class + 1/2 their other character levels. This makes multiclassing significantly easier.
The upshot of these changes would be more flexible spellcasting that could feel different for different styles of caster.
I wouldn't be averse to putting spells into narrow schools like the Bo9S styles, but that would be a lot of work.