A user on twitter asked the boys on the Diablo Twitter page how many classes they planned for, and if the locations visited would cover all those detailed on the Sanctuary map.
Their response, was thus...
Official Blizzard Quote:
Diablo: @Midnightact We'll have five classes and no we're not creating the entire world in. We're visiting specific locations similar to Diablo II.
The part about the classes, we all knew.
And the part about the locations, may have you thinking "Oh joy, a vague response."
But bear with me.
The locations in Diablo III so far, have been a forested lowlands, and a desert. The locations in Diablo II, started with forested lowlands, and then moved on to a desert.
If we take that trend, then we can pretty safely assume that next up, we'll be seeing a jungled area, and then, we'll be travelling to hell. This speculation of Hell's sequence in the acts is strengthened by how it appeared in Diablo I, and Diablo II. Both were reserved for the last act.
If this isn't enough for you, remember those bubbling waters we had to trudge through in Act III? Well, some time ago, they tweeted about work being done on a tar pit effect. Discussion ensued, and some suggested that it would follow in the footsteps of the bubbling waters of Diablo II's vision of Kurast.
While we probably won't be visiting the EXACT same areas as in Diablo II, it does seem that the areas themselves, and the sequence that we've seen them in, is quite similar to how they appeared in Diablo II.
Anyways, time has yet to tell what the next act(s) will hold for us, but hopefully, this gives us a hint at what's next.
Or, if you don't agree with my interpretation of this, then check out Scyberdragon's, located here.