An MMOFPS where multiple factions were vying for the same solar system, made up of 5 planets, a few moons, and an asteroid belt. Everything would be in real time and the main server would be run on an IBM supercomputer to keep up with the demand.
It would be in the vein of ArmA, where it was a combined arms simulator. However, Space would also be an important factor, where space superiority over a planet would mean the ability to use dropships and orbital bombardment of enemy troops. There would be classic sci-fi fare such as mechs, but there would be a focus on projectiles over particle weaponry.
now, there would not be simple inter-species fighting. There's always been either various factions of humanity, or various species fighting over the same area. Instead, there would be multiple factions AND multiple species fighting over the same area. Even if one faction gains the advantage, you can band up with other species, or unite your species, to push back. Combine this with the fact that you need the resources and production capacity of planets, moons, or asteroids, and you have a chaotic war where no side is truly united and backstabbing is commonplace.
Tech would be mostly balanced, but species would have certain unique advantages. However, if you captured an outpost of an enemy species, you would be able to use whatever tech they had left over. You could also have access to it if you allied with a species. Each would hold specific advantages and disadvantages. If you did FINALLY manage to unite the solar system under your control, you would have to face other things, such as insurgencies by the other factions. They would most likely succeed, but it would be a tough fight for them. In all, the uniting of the solar system would be a grace period, where you would be able to build up your reserves and have the force to (somewhat) fight back a revolution from all sides.
The game would those commanding the tactical, and those commanding the strategic. There would be tanks, mechs, apc's, fighters, bombers, dropships,and howitzers. In space you would have frigates, corvettes, even mighty carriers and destroyers. And of course, a wide variety of tactical weapons, including venerable assault rifles, heavy hitting combat and sniper rifles, machine guns, shotguns, sub machine guns, rocket and missile launchers, flame throwers, and a variety of other weapons, like acid launchers, mortars, personal lasers (handy for burning a hole in that tank tread) and more.
In short, the best Combined Arms Simulation there ever was.