if you use 3 bitmaps to define a landscape instead of one you can get beutiful results, like for example, tunnels or houses (with windows and all) or bridges...

this gives you the advanges of a voxel engine (doing comlex scenes with low CPU) but not limited to only a simple landscape.

you can have your artist make a wonderful scene of hills and tunnels and houses with thousands of polygons in a 3d program and convert the polygon scene into a 3-layer voxel with a simple algorithm (although slow). the result would be (at least to my imagination) an amazingly comlex scene that can run pretty fast...

then again,, i could be wrong, maybe more than one layer would make the algoritm too slow. i dont know.