I was wondering about this today after a discussion with some friends and took the liberty of Googling it to see if there were any details released. Unfortunately, there aren't any, but I did get this thread idea from others I saw.
What would you like to see in Diablo 3? I'm thinking everything - acts, story, gameplay, classes, the whole shebangabang.
I was thinking it'd be cool to have the game in actual 3D and let you explore with the camera set behind you, and have a quick button so you could switch to the isometric camera angle to make combat easier.
Also, a small thing like adding male/female options for the classes. I think the classes from Diablo 2 would be good for the carry over, but maybe new skill sets or a different skill system entirely. (they did it from D1 to D2, why not this one?)
I need to stop thinking.