"expand subfolders when the pointer hovers and waits over a folder that contains them, while doing drag, ready to do drop."

While the user is working within a folder, he may have made selections ; at the time of dragging the selected contents to a folder at which to move them , it may not be visible because it is subfolder and in "non-expanded" mode. The most normal thing is that while the pointer ( with the "transparent" status of the contents being moved ) is ABOVE the folder that needs to expand to show the subfolders, in this precise condition, the expansion is automatic and allows the files to be moved to the finally visible subfolder.
Otherwise, other workarounds or other systems are needed. The more "manual and immediate" one is, or appears to be, impossible.