CAVEAT: This screed is still in progress.
In D&D, all classes contribute to fighting. A wizard contributes +0.5 per class level, on average, to BAB. A cleric or rogue, +0.75 per class level. A fighter, +1 per class level.
Similarly, every class contributes to saves.
I propose that all classes also contribute to caster level. Moreover, I suggest that caster level is not tied to class. A wizard/druid should have one caster level, NOT a wizard caster level and a druid caster level.
Just as some classes have weak attacks, some classes also have weak casting. Fighters, rogues, and barbarians contibute only +0.5 per class level to caster level. Paladins, monks, and rangers (and maybe bards?) contribute +0.75 per class level. Druids, clerics, wizards, and sorcerers contribute +1 per class level to caster level.
This common caster level would be used for all classes for all the usual suspects -- determining effects such as duration, range, and damage for some spells. Als for caster level checks to dispel magic or penetrate spell resistance.
I also propose divorcing spell DCs from spell level and instead tying them to caster level. Spell DCs would be set by:
10 + half caster level + ability modifier + other modifiers (spell focus, for example)
(Arguably, this is more powerful than current DCs, but it is elegant and follows the model of D&D mechanisms such as armor class. Still, one could change the "10" to a "9", an "8", or a "5.")
For a single class caster, this makes low level spells more useful. A Grease or a Glitterdust becomes effective against foes with good saves. So it is a power booster for casters. But that is not what recommends it.
It is also a return to the earlier editions of the game, insofar as making a "save vs spell" did not vary with spell level. But aside from a perhaps debatable return to tradition, this is also not what recommends it.
The value to this rule is its potential to help multiclass casters.
If one's caster level is the sum of all caster levels from all classes, then the multiclass caster, although still penalized by a lack of high level spells, at least can cast low level spells effectively. The trade-off as a multiclassed caster is to gain a large variety of spells and a large number of low level spells but to lose access to high level spells.
This is part four of an ongoing series on How I Would Fix D&D 3.5.