With all due respect to those who posted ahead of me, we need to think about how each act looked, and how an expansion can be placed in to make things look different.
Act one is New Tristram, which looks much like a superstitious village on the sticks of an old and abandoned monastery should look. In my opinion act one's setting is that of a haunted house horror aspect.
Act two takes on a different aspect with the city of Caldeum, it is bright and desolate, so to make use of it's horror aspect is to have things jump out at you from nowhere, to use the terrain against you, which is something we see from an assortment of monsters.
Act three again gives us a new look, now instead of being in a creepy village, or a desolate but spacious environment, we are placed inside a keep under siege. Bastion's Keep plays a different role of horror to the player, with limited space to move about, we feel claustrophobic with monsters climbing the walls and spawning in great masses.
Act four acts on the role of perverting and soiling halls that one never thought could be fouled. Act four may lack some of the horror aspect as did the three previous acts, but its scenic splendor, and befouling are candy to the eye.
If there is to be an expansion, one needs to think of ways to add new scenery, to continue with what has come before. There will probably be aspects of a new element of horror to behold, new environments yet to be seen.
My predictions are thus, the next act will have a textural layout similar to act three of Diablo Two, water will play a major role in this forthcoming expansion, there will be bridges, in a dark atmosphere which will make it difficult to peer ahead, narrow terrain with more accessibility to the monsters than to the player. Unpredictable monsters that attack from right on out of the environment, dangerous plants, and water creatures, all in an Asian horror mystique.