Ohhh...Why not map? I've tried to modify COS map and units. It was possible,inside borders, of course. COS's rivers are strongly questionable, f.e. What about Kerch Pennisula and Crimea? What about Crete?
OOB could be changable, too.
Some example of private modification ("chrome"):
I've changed nationality of Polish Air Force to British, so it could be reconstruct after collapse of Warsaw. The same with weak Polish mobile unit "Maczek" (strength 2/MP=5).
I've add Czechoslovakian Corps to Soviet OOB, as well as Polish People's Army.
I've added British and Italian airborne units to allow airborne operations against Malta and airborne preparing of amphibious operations.
BTW - there was basic problem in COS, I hope resolved in CS. There was no possibility to transfer UK/USA air units to Balkans, without earlier conquer of Italy or Turkey. Churchill's strategy was just impossible.