Firstly, if of course this hasn't yet been suggested before, will you be able to use your advantage in naval power to create a blockade of a port. This could either reduce the unit's supply capability, or its effectiveness in battle that is in the adjacent city or area.
Secondly, If lets say the Unites States had a surplus of war materials (ie MPP), and the USSR is in much need of supplies, like it was after its demoralizing defeats throughout 1941, can you transfer them among each other. Also, Germany could make an effort to prevent this from happening, by itself harrassing sea routes and naval traffic.
Like mentioned before, I havn't quite had the time to skim through 11 000 posts to see if this has been mentioned before, so excuse me if this is old news.