Wouldn't it be cool if there could be player-controlled turrets, I mean like a player can hop on the back of it and chooses what to kill. Kind of like the mounted machine guns on Cod/Halo only with more awesome such as cluster grenades and high powered laser beams. Just a thought...
Basically a deployed gunner or tank is the closest thing to being on a turret. Gunner is fun sometimes deployed especially against a noob group.
They had a feature like that in Mechassault 2: Lone Wolf, the successor to the original Mechassault(my personal favorite and best game ever made, as far as online play and community). You'd fire a pretty powerful missile and control it like a self propelled grenade, useful for killing campers or hidden things but got really really boring to me. I don't think that would be a viable option for this game but still a neat idea.