3D terrain would need much more complex code to render, and I suspect it would likely be too slow without low-level support. The current code only supports a single textured ground plane, and it uses simplified math to speed things up in ways that are only valid for horizontal planes.
Adding a few untextured polygons would be possible, for example combining it with the renderer I used for Space Rocks 3D and Carrier 3D, but general textured polygons would be a lot harder.