I'm considering a campaign based on Bulfinch's Mythology. There are three settings to choose from:
1. The Age of Fable: Mainly Greek Mythology, but includes a short section on Norse Mythology.
2. The Age of Chivalry: Arthurian Legends and accounts of notable knights in history, as well as some tales of Robin Hood
3. Legends of Charlemagne: A fantasy version of Charlemagne and his Paladins.
Most of the PCs' opponents will be human or animal, but the tales also include monsters which are included in most fantasy RPGs. The PCs themselves will be human, but most of the core classes would be available.