Battlefield 2 sortof took a crack at it, with it's commander role. Ofc it wasn't much RTS, since every one of his "units" was a player, who he could only give orders, without knowing wetehr or not the player owuld follow them.
He could spot enemies, and send in vehicles, ammo and air strikes.
As for the mercenary thing, you could do an RTS in the style of DotA, Heroes of Newerth, League of legends Demigod.
In those games all the normal units are computer controlled and just follows predetermined paths, and has some aggro rules, and the players each controlls a hero, like the heroes in Warcraft 3. They'er sortof like a mix of RTS and combat RPG (the don't have teh role playign elements, but they have the leveling and gear elements of RPG's scaled down)
But i don't see why you couldn't have one player controll all the creeps (the AI controleld weaker units) on each team, and have heroes controleld by players as well. That way one guy on each team would be playing RTS and x otehr players on each team would be playing solely their hero like in the games mentioned above.
Demigod had a feature, where players could use their money not only on new gear for themselves, but also on upgrades for the team, like shorter respawn rate, thougher buildings, tougher creeps, more gold income etc. Stuff liek this could be handled in a more RTS-ish way by a single player controlling all the non-hero action.
Heck, you could even let some heroes be played in first person with ranged weapons to please the FPS players. I seem to remember that in dungeon keeper 2, there was a spell taht allowed you to directly controll one of your minions from 1st perosn view, so i don't see why it shouldn't be possible here. You'd just haev some sort of cooldown on your attack, like a lot of weapons have in FPS games.
Dunno how to turn this into a single player thing though, but i guess thats redundant anyways, cause if they wanna play RTS, they could just pick up an RTS, and teh same for otehr genres.
Only problem with it not beeing single-player friendly i can think of atm, would be that story would suffer greatly. As all the above mentioned games are pvp games (or player vs AI) and are played out in rounds, so having an story more complex than faction A hates faction B so they fight in them would prolly be kinda hard.