If I could do full war combat game I would have 3 separate games that cross connect to each other, but still maintain their own, unique environments.
It would be sci-fi based because they're easier to play balance (nobody has any pre-conceived notions on how something is supposed to work), and to get some good "naval" combat using starships that do not take days of real time to play out.
The different layers are:
1. The solar system, for space battleships to blast each other and weave between planets at faster than light speeds. If you need a point of reference, then think "space Jutland". No, I don't use space fighters. Fighters fly in the air, while warships sail on water in real life, but in space they're sharing the same medium. That means they're not really fighters, but 1-man PT boats. Besides, I don't want warships to be second rate in their own theater of war. I like battleships and so outer space is a battleship vs battleship game.
2. The atmosphere of a planet above a battlefield for an area dozens of kilometers in size, for fight planes to dogfight in a true flight sim, yet has simple ground details of just low res trees and buldings. It would be a joystick controlled game very much like playing a stealth jet fighter game with radar and short range missiles, yet easily spoofed and dodged to promote some dogfighting. You have to use your eyes since you can't see very far with "radar" or whatever they're using. The closest game I can think of would be Wings over Vietnam, but replace the Phantoms and Mig-21s with sci-fi fighters. I would also add that you can break the sound barrier, but you are detected. This is to explain why jets are optomised for slow and stealthy, instead of fast and not armed any BVR missiles.
3. The ground fight in a single, highly detailed square mile of the ground that plays out similar to BattleField 2142. Yes, you would have helicopter like vehicles, and even floating tanks, but not any fast, jet fighters. I would even have power armor suits you can use, but in my case I'm partial to copying something like the Cyclones from Mospeada/RoboTech.
The interaction between the various games would be things like a ground combatant requesting an air strike. When that happens an automated request for laser guided bombs to be brought to a waypoint on the air map is added. If you fly a plane loaded with LGB's to that spot, then the laser spotter on your belt in the ground game is activated for use.
It would also have the ability for the ships and planes to interdict transports bringing cargo to the ground battles. The less supplies that make it there, the less often tanks, power armor, and heavy weapons will respawn back at the home base for the ground guys to use. In space they're AI cargo ships heading for the planet. In the air you see convoys of trucks from their side of the map head to the middle of the map where the "ground game" is represented.
The reason I think having separate arenas for the different types of combatants is they do really have different needs and requirements for how they play. Games like Arma-2 and Aces High do have both ground and air assets in one game, but they sacrifice things to get them together.