Mid-surface extraction would indeed be nice. In FreeCAD it’s completely manual now. You can use the Facebinder and then 3D Offset tools to do the extraction itself but then the tricky part is to connect the extracted surfaces properly (typically by extrusion of subshapebinders based on edges).
I know that Matej planned to have a look at the possibilities of implementing some partitioning (Adding a feature to split edges of geometry) and defeaturing tools (the latter are really powerful in FreeCAD though: STEP file Cleanup) in the future.
Creating new geometrical features (even simple ones) would be much more problematic. Especially since primitives are usually insufficient.