One thing I thought of is a sort of 'nightmare' act. Not the difficulty but a literal nightmare thats filled with demonic stuff. Designers wouldn't be limited by 'normal' reality there either so you could have some cool stuff happen.
Another idea is maybe a sort of nordic forest that has scatterings of little towns filled with cults. Pentagrams out in the forest. Churches burnt down. etc...
The blizzard north version of diablo 3 looked like it had a heaven type act. That could be interesting, Maybe your helping angels fight off demoic invasions. Its would be interesting to see an artistic interpretation of heaven and to fight along the angels like tyreal.
As far as settings that should make a return... I think the jungle is fitting since it goes so well with the witch doctor and the whole voodoo thing just seems very demoic. The corrupted/satanic church thing (monastery) is also too good not to bring back. Obviously hell is fitting for a return :)
What are some of your ideas/thoughts for what the other acts might be or ideas for good diablo acts?