Or, do they have to be human? How about a kind of creature with a hive mind, like Zerg or ants/bees/etc.? Or robots where the mind/soul/personality is stored on the server, not lost when a body dies; the body is just a remote labor unit.

Yes, they pretty much have to be human ;). With all the Call of Honor Modern Ops around, I'd like to explore the very human feelings you get when you are sent out in a world where everything is here to kill you, where your life is essentially in the hands of the guy next to you. My "hidden theme" if you'd like. Doing this with anything else than humans makes this harder, or impossible.

Your character (who is also the main yet expendable protagonist) kills all in the way, as per his orders and encounters in the end... himself

I love that one, but it's not a pure singleplayer game, I don't have one protagonist. My Protagonist is a group of soldiers, the players. I may not allow only one player to play the game.
Awesome, but this idea is better suited for a different game.

Or, do they have to be human?  How about a kind of creature with a hive mind, like Zerg or ants/bees/etc.?  Or robots where the mind/soul/personality is stored on the server, not lost when a body dies; the body is just a remote labor unit.

Yes, they pretty much have to be human ;). With all the Call of Honor Modern Ops around, I'd like to explore the very human feelings you get when you are sent out in a world where everything is here to kill you, where your life is essentially in the hands of the guy next to you. My "hidden theme" if you'd like. Doing this with anything else than humans makes this harder, or impossible.