I have a -small- list of possible suggestions:
-Buggies with a machine gun, all controlled by one player
-more helicopter types, and also when a map is fully Under one's control, have a chinook-style chopper dropping most of the other team on a random spawnpoint...
-more ammo boxes/ or they can replenish themselves-small patrol boats or other warships
-customisable soldier (and bots) -> helmets, webgears, backpacks, haircuts, ... (as an option)
-choose team colour option
-drivable tractors option
-a no-air vehicle option-a webgear that adds two more (or one more) small add-ons (at the cost of one spot, or the side-arm)
-add-ons for weapons (scopes, bipods, grenade launchers,...)
-crawling key
-more guns, sidearms included
-knives
-scouting drones
-automated turret
-placeable/pickable machine gun on tripod (you place it, shoot, pick it up, move,...)
-a melee attack button, to hit 'em with the gun's stock
-...and if you are crazy enough, the ability to create your own vehicle designs, with a creator function similar to spore... (this is the weirdest, crazyest idea I could come out with...)...
-a basic crosshair
-player-hosted servers, and LAN games...
Still an awesomely great game, but I still give my -exessive- suggestions...