I think the best system of military control in HOI4 would be a kind of theater system, where the major nations could control all forces in a particular theater of operations.
This would resemble actual WWII history, with say the British commanding forces in the Mediterranean under Alexander, and the US controlling allied forces in France, under Eisenhower. other examples would include US and Australian forces in SW pacific under MacArthur, or allied forces in Burma under Slim (UK). For the Axis, Germany in command of all forces on the Eastern and Western fronts, and Japan in the far east, Finland controlling forces along the Finnish front, and Italy in the Med. I know it may be too simplistic an explanation, and that there were often cooperation between nations rather than direct control by a single command structure, but I think it would be a good representation. This would allow the player to focus on particular areas more effectively, rather than having a random mess of individual nation's troops doing their own thing. I'm sick of the commonwealth doing nothing, building only tactical bombers and not being able to defend themselves, let alone send assistance, or the US sending troops to random, suicidal locations. I hope that we will be able to have a game where the western allies do something and attempt a Normandy-like invasion of Western Europe. For some reason in TFH, all they do is attack Greece and leave Italy and France alone. Maybe having a simpler theater system of control will help to rectify this, or at the very least, make it more accessible and fun for the player. Each theater commander could say, request troops, war material, naval and air support from an allied pool of resources. depending on the availability, certain forces will be released to these theaters, with the strategic needs of the high priority theaters being met first with more troops, support and newer equipment. For example, the European theater would take priority, with massive preparations for an amphibious invasion, the Mediterranean theater would come second and Burma would come in dead last. By assigning a single nation to take command of each theater, under a certain commander, this would help to streamline the game, making it more realistic and hopefully more fun. Ironically by giving each nation less control, they'll have more presence in game. Hopefully we'll see NZ troops in Egypt!!!