An interesting idea which I think I saw proposed by Colonel Hardisson some years ago was a campaign were magic power was less available - nobody could be a single classed caster. All casters had to be multiclassed, and couldn't have more caster levels than non-caster levels.
This is quite an interesting idea for a 3e campaign where the most epic wizards at 20th level are Ftr10/Wiz10 or Ftr10/Cleric10 or Rog10/Wiz10 etc. It means that the really powerful 5th level spells (teleport, raise dead, magic jar) only enter the scene when PCs are about 18th level (9/9 multiclasses), and everything 6th level+ is just unavailable. Fireballs or Fly don't appear until 10th level at the earliest.
This could be an interesting campaign specific way of limiting the power of wizardry or clericism in a campaign.
Cheers
Another mechanical means to a similar end in d20 is to throw out all casting classes except the bard, but let the new "bard" get wizard, druid, etc. spells. If your class is Bard (Wizard), you still cast mechanically as a bard, but your spell selection can include things like fireball--eventually. With a bit more work, you can even keep the mechanics but impose tighter limits by theme--for example, have a group consensus that if bard(enchanter) means a small subset of cleric and wizard spells. Since the 3.* bard is already locking into spells, a certain amount of thematic picks are already likely, if only as an emergent property.
This works even better in 3E than it does in 3.5 or other later variants, because the 3E bard being so weak is not at all hurt by getting a boost via a (potentially) better spell set. You still have a "sweet spot" where the bard eventually dominates, but it is so much later than standard as to be a non issue in many campaigns.
Personally, if I were to run a 3E campaign like that (and I sort of did a variant of it with Arcana Evolved, briefly), I'd bring in the 3E bard (or tack the 3E bard spell progression onto a similar class) in Arcana Evolved, and ban the rest of the casting classes. I'd use AE because, while not perfect in this respect, Cook went to a great deal of trouble to make spells that boosted other capabilities instead of taking over. The low-level charm, for example, make the Diplomacy skill better, instead of replacing it. With only "bard" casting, and even with the extra flexibility of the AE magic system, you could run that kind of AE game up to about 15th level or so before it started to break down.