Heaven could be a possibility and Hell wouldn't be too bad. If you check out some of the artwork, you'll see some quite unique places. There's one that's a maelstrom with a wizard/person standing infront of it.
Here ya go:
http://www.blizzard.com/diablo3/_ima...twork/ss58.jpg
Also on the World map there's Ureh, the City of Shadows. Knaak wrote a decent book about the city. Here's some artwork that matches the descrption of the book and it's cover perfectly. It'd be a great act/quest/end game content:
http://www.blizzard.com/diablo3/_ima...twork/ss47.jpg
http://www.blizzard.com/diablo3/_ima...twork/ss59.jpg
This is the Amazon home, but Baskiok said it won't be in the game. They just included this artwork to provoke some thought about how large Sanctuary is.
The world of Sanctuary is quite big, so I'm not sure we'll fully know until the game is out. Also it may not only take place in Sanctuary.