Since I was a wee kid, I wanted to see a game that mixes two genres - RTS and (T)FPS. Not like Command & Conquer: Renegade, but actually has the two genres in a single game. You have a mission based campaign just like in Red Alert or Tiberian Sun, but once you build up your base and send your troops out, you can choose a unit to control, any unit, and that's when it becomes Renegade. And it's not like it turns into some singleplayer campaign, story-driven story. It's a sandbox, you can seamlessly go back to the RTS view, order your troops around. You get mostly the capabilities of the unit, its HP and everything, when you go into FPS as the unit has in the RTS. So if you turn into a basic infantry, you're expendable, and the only real difference is the player skill (and you can jump, and avoid enemy fire, take cover etc). If you go for the Mammoth Mk2, you're a slow AT-AT style fortress that obliterates, but has no air defenses. If you go for a stealth bomber, you're that. Obviously, it makes the best game sense to choose the important units that can make a difference, but you can also win by just playing the RTS well and playing basic infantry for fun - it's not a requirement to play the FPS to win the game.
It makes no economic sense, because you'd be making a second game alongside the RTS (or vice versa) that some players will not even see, let alone appreciate. But I would love to see a game like that.