This exact idea has been done, precisely the way you described it (the first idea, not the mercenaries one). It was called Savage: Battle for Newerth, and I personally loved it, and I usually don't like RTS's, so I suppose it did exactly what it was designed to do.
Other opinions may differ, but I still remember Savage as a unique playing experience, several years after playing it. They released a sequel which didn't take my fancy, and I have no idea what's happened to the series now, but I for one hope somebody takes up the idea and runs with it.
The key point is, they mixed genres, and as far as I'm concerned, it totally worked. So, you should consider giving it a go, since it's totally free to download and play now.
EDIT - Here is the link to the game's site. http://www.s2games.com/savage/
It also had a lovely map editor that led to it having thousands of different maps, many of which were spectacularly huge and supported a full 64 players beating the crap out of each other, directed by a single commander and elected officers. Definately one of those gaming moments that stay with you.