I think it would be great if you could apply “transparent masks” to cells, i.e. bricks, terrain, waves and the like. Perhaps even an option to create these yourself, like in bitmap editior, would be fantastic!
This way you can create bricks in different colors, lines, shapes and even furniture. All you need to do, is place the cell as usual and then apply the transparent mask on top. This addition would open up a whole new layer of options and allowing you to share your masks with other users, would make them/us do all the ground work.