A new pantheon sounds nice... Kind of hard to implement anything about a pantheon in DC right now, though (unless you sacrifice the cleric and druid to make two slots for a pseudo-specialty priest)...
Having more class slots would make this a bit easier in the future, I guess (but first things first, i.e. making healing spells work outside of combat, and then making enemies able to flee normally, and then making enemies able to use magic/abilities).
Oops - forgot a few other campaign worlds: Lankhmar, specific historical culture (Vikings, India, etc.), etc... Will update the list above
One of the better historical campaigns was the Viking Campaign; using the 2e sourcebook, a few Dragon magazine articles (detailing beasts such as the linnorms), and the Legends & Lore deities, and you have a very viable, interesting campaign! Unfortunately, 2 of the 3 sources mentioned here are in my oft-mentioned stash in NJ... Alas...
Lankhmar was a pretty interesting campaign as well, although not as supported as the more "main" campaign settings like Forgotten Realms or Greyhawk or Dragonlance... They have their own magic systems, too...