Not everyone knows Battlezone 2. B minus rated sequel to highly acclaimed Battlezone, a vehicle FPS + real-time strategy game. Build a base, grab resources, build units, send units and yourself into battle.. with yourself controlling a vehicle. More of a vehicle sim mixed with a RTS than a FPS. You could also go into a "satellite view" and get into your God-mode view.. like Starcraft. Send troops here, attack this, defend that.

Idea: Instead of vehicles, build soldiers/monsters/whatever. Instead of huge open-terrain, have both sides fighting each other in your typical hallway complex. Go into "god view", group your soldiers into squads, send them into battle. Patrol this, engage these guys, guard this room.

The game Sacrifice pretty much got the idea. But if i'm not mistaken, most of the combat takes place on huge open terrain.