I think adding an index output for selected sub objects is a good idea, but, using this index output in other nodes brings a new challenge:
Some commands like fillet, chamfer, offset, blend and extrude behave differently with curves, edges and faces.
For example, I'm not sure how to distinguish an index for a node like "offset". should it behave like an edge/curve index or a face index or an objectlist index ?

I made a "Select_SubObjects" node. It has two outputs (for now). One output for edge/face sub objects, another one for extracting selected sub objects as separate objects.
It works nice with extrude and shell nodes without needing to change anything.
Also, adding a corner point output for fillet/chamfer is another good idea. Thanks :)