Looking at it with some perspective, I think we already have Monte´s suggestion working. Somehow. A Druid is a cleric with less spells and a few supernatural gimmicks -wild shape, thousand faces, etc- The analogy works better with the PHB II´s Shapeshift variant.
I think an option lke that would make high level casters more playable. If we had casters with a bunch of always on magical abilties and a reduced number of relatively potent spells, the process of making those huge "spell prepared" lists would be much more easier. Suppose a wizard´s spells per day wouldn´t increase past, say, level 6. He´d have 6-8 prepared spells, and a number of magical abilities that would cover those virtually always on defensive and offensive spells; a high level sorcerer, for example, has so many slots that the number of Magic Missile and Mage Armor spells he can cast is, for almost all practical pruposes, unlimited.
6-8 spells don´t seem to be so many, but the number is in line with the number of slots a wizard has in his 2 or 3 higher level spells. In my experience it´s when those slots are expended when the wizard starts thinking he´s "empty"; once he´s cast his Mazes, Empowerd Disintegrates, etc, his offensive options dwindle greatly in power. Also, a reduced number of slots would surely mean keeping buff spells under control. The number of temporal modifiers is a big factor in the complexity of high level play.
Pairing those powerful spells with useful, comparatively magical effects we´d get a caster more customizable, about as powerful and is now, and not much more difficult to play at later levels as he´s at low-medium levels.
The only problem is that he´d only prepare his higher level spells, relegating those he adquired at lower levels to oblivion. But what if those spells augmented freely, like this example?:
Fireball
Evocation [Fire]
Minimum Level: Sor/Wiz 3
Components: V, S, M
Casting Time: 1 standard action
Range: Long (400 ft. + 40 ft./level)
Area: 20-ft.-radius spread
Duration: Instantaneous
Saving Throw: Reflex half
Spell Resistance: Yes
A fireball spell is an explosion of flame that detonates with a low roar and deals 1d6 points of fire damage per caster level to every creature within the area. Unattended objects also take this damage. The explosion creates almost no pressure.
You point your finger and [snip].
The fireball sets fire to combustibles and damages objects in the area. [snip]
At level 11 you fire an additional fireball. at level 17 you fire two additional fireballs
At level 14 the proyectiles fired grow to be 1 foot in diamater; you can make a ranged touch attack with them to deal 2d6 blugeoning damage. Creatures struck by the pea suffer a -4 penalty to their Reflex saving throw against the spell´s fire damage.
Material Component
A tiny ball of bat guano and sulfur.