It would be great to display 2D-drawings on any 3D-plane - think of a story with the planview under it.
I personally would also like to draw with all 2D-Tools on selected surfaces in 3D with all functionalities.
Now - morhphlines are the only way getting lines in 3D - but no real linetype-integration, no linewith is possible.
Drawing hatches directly on walls in 3D with the possibility of space-klick to automate this like elswhere in Archicad and the possibility of getting the area-m2 like in 2D - wow! It could change some workflow..
Maybe. AutoCAD and many others CAD do that. But ArchiCAD was designed differently - where 2d and 3d is strictly separated. To do what you are offering then should change basic concept of ArchiCAD workspace.
You can always save your plan as DWG and open it as an object - to show it in 3d.