An online shooter/flight sim/third person action/RTS game based on Starcraft. Bear with me here, I've had this idea since about 2001.
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Basically, anyone who wants to can log in and play Starcraft as normal, against other players, CPU armies, custom maps, et al (minus ladder and league play... there's no way to keep this truly balanced). Anyone who wants to can also enter the game as a hero unit (an upgraded version of pretty much any combat unit in the game). Players would level up their hero units playing first-person within a existing Starcraft matches. Death for hero units would simply kick the player out of the match they logged into (and ban them from it).
Of course, whichever side they fight for would need to pay them to have them enter the game on their side... otherwise, they're a neutral (hostile) unit to all RTS players in a match. Any orders given to a Hero unit appear as objectives in that player's HUD... they can choose to ignore orders, but the RTS player can also choose to boot 'em out of their army and take back their pay.
In order to keep people from ganging up and completely unbalancing the game... anyone who logged in as a hero unit would be anonymous to all RTS players, appearing only as Hero, (unit type),(unit stats). Likewise, Hero unit players looking to log into a game could only see how many RTS players are involved, the map they're playing on, and how long the match has been going on. There would be no communications available in the Hero unit players' lobby, and regular old school battle.net communication available in the RTS players' lobby.
Mind you, there would need to be a new version of Starcraft with a much more complete physics engine to make this work... and Starcraft 2 ruins my whole concept >.<