Keep in mind, this is just an idea and I have no coding experience to know if it is possible at all.

Every spring phase, each unit has two different move orders. An actual order which the real unit will follow and a phantom order that a phantom unit will follow. Thus, from every unit on the board, two more "units" will be spawned, a real one and a phantom one. In the autumn, the real unit and phantom unit each has its own order instead of having a real and phantom order for all units again. At the end of autumn retreats, all phantom units are destroyed and only the real units remain for the counting of units and SC's to determine builds.
In terms of some of the specifics for gameplay, phantom units have a strength of 0 and real units have a strength of 1 (like normal diplomacy). Phantom units act exactly like real units, except they cannot capture SC's in the autumn and their support holds or support moves do not add support (because their strengths are 0). For example, a phantom unit supporting a real unit against another single real unit would result in a bounce because the phantom unit adds no real support.

A couple questions I thought of that I figured would be good to have input on:
What happens when a phantom unit is attacked by a normal unit? should they be allowed a normal retreat or should they be disbanded automatically (like non-player units in variants like Known World 901)?
what should happen if a phantom unit attacks another phantom unit? Should they just bounce or should both be destroyed?
Should it be played on a smaller map? or be adapted to an existing map?

All in all, this is just an idea I had and wanted to put it out there to see if it would be a plausible variant idea. Please give me your input :D