Norworld? Where high level PCs relocated to build their domain.
Karameikos? Beginning your career in a culturally divided nation where a neighboring empire has colonized. Locals are resentful, restive, and rebellious.
Glantri? "Alpine" nation ruled by wizards, clerics are outlawed.
Darokin? Merchant-based realm beset by the Master of the Desert Nomads.
Northern Reaches? Vikings . . .
Minrothad? Sea campaigns . . .
Thyatis? Eternally doomed struggle against the dreaded wizard-ruled empire of Alphatia.
Ylaruam? Arabian nights long before Al-Qadim was dream.
Ethengar? Adventures among the Mongolian horde . . . long before FR thought of The Horde.
Sind? Adventure in the mythic Indian subcontinent.
Savage Coast? Find the cure to the Red Curse in a world evocative of Earth's Age of Exploration.
Hollow World? The Immortals' extinct people museum, ancient history without the time travel conundrums!
Shadow elves? Albino subterranean elves, and not necessarily evil either!
It's all in one world. So?