This suggestion has been made before and you are certainly right about the commercial possibilities. But it is not so easy...
I bought Scourge of War soon after it was released - I thought the graphics were wonderful as you see every building, farm, road and tree around Gettysburg. HW has never had that sort of detailed feel but of course it is designed for something different - the ability to simulate many battles and to give a top-down command experience. SoW, on the other hand, gives a wonderful feel at the regimental level but I find it confusing at the higher levels.
I think there would have a huge rewrite of the HW AI to make a rival to SoW (two simple examples, a) HW ignores roads and SoW makes them crucial to troop movements b) SoW shows you couriers moving across the map and depicts the individual riders approaching officers to deliver orders - HW has never gone to that level of detail).
HW has followers who are experts on the Napoleonic era and SoW has the same sort of expertise. For JMM to impress the Americans, he and his team would need to become experts on the Civil War - It would be asking a huge amount - he may not even be interested in it. (I would be very excited if he went backwards in time and covered the 7 years war or the period of Louis XIV but I don't think it will happen in my lifetime).