Something like the US Predator or UK Phoenix ...
You would need a control panel which triggers a live cutscene of what the UAV can see when the player moves near it.
Might be fun in MP ... program the UAV to loiter where you think the opposition are and hope they're not looking up.
UAV's are often rail launched from the back of a truck, I don't know how difficult this would be to model.
On a related note, has anyone come up with a way of implementing any type of remote controlled vehicle?
Ade