The next step for isometric rendering would be to create the foundation of an extreme detail design tool. This would allow previewing the final result while drawing height maps or transforming raw vertices directly.
First import a standard model format using only the vertex data. Then create an algorithm for reducing the triangle count for the shadow casting shape. The tool could do the entire sprite rendering step as a preview, but exporting to a model file and separate rendering script would allow automating the process if the settings change while making a game.
A simpler triangle rasterizer optimized for small triangles and progressive detail previews based on performance can make the isometric sprite generation fast enough to be used in real-time.