some way to drop the last satchel out of your inventory
trees that seem to become transparent to ai when they are knocked over (don't try using them for cover)
partially steerable parachutes
pop up menu commands programmable to keyboard or secondary usb keypad (switch weapon, get in driver...)
spec op cameras (all sorts)
improve radar and infrared if you don't turn on the engine you shouldn't show up for heat-seekers, likewise with radar if its off they shouldn't be able to lock up so easy
Air to air missiles and ability to select different loadouts. Not elaborate, just air to air or air to ground
ability to commandeer vehicles from ai units and civilians (warning shots?) without killing them.
severe in game penalty for team killing: offender taken out of action for a bit, or severe (emotional) injury
oh ya, and mimes