VEHICLES/MECHANICS
Vehicles have stereos that can listen to the "pirate" radio stations that pop up.
Ability to use binoculars in a stationary vehicle
Various size tires for the various vehicles; as in large tires only go on Urals, med tires on standard cars, small on bikes, etc......
Ambulances
Bullet proof glass (within reason) that can be found at crash sites
Bike locks
Some vehicles have car alarms that you must be careful not to set off (or set off on purpose because someone is near)
Passengers can blood bag each other if in the back seat
Player does tuck and roll if ejecting from moving vehicle
Railroads w/ working trains
Vehicles can be covered by brush and shrubbery
Horses
Police Cars
Vehicles can be locked
Some sort of hijacking mechanic (I jump in your car and hold a gun to your head, passengers can retaliate if they are present)
Tandem Bicycle
Can attach flood lights
Can attach megaphone to yell at people from the car (direct chat shouldn't really work while in a vehicle except to those in a car....forced)
Cannibalization of parts
Spotlights on some vehicles that people would control from the bed of a truck.
If heli pilot is shot, co-pilot immediately takes over. (there is two joysticks after all)
Manual train carriage
Row boats
Doors actually open.....can be used as cover
Tandem bicycle
Wheelchairs
Flatbed gasoline trailers
Car batteries that run out
Car batteries that can be removed so your vehicle is not stolen
Jumper cables for car batteries
Car keys
Hotwiring (with correct toolkit)
Certain parts require two survivors (because of part size)
Two survivors speed up tasks that could be completed solo
Towing
Can be painted on
Can be written on to leave messages (if you have the propper item)
Armor plating (need a welder or some tool)
Passengers can shoot from moving vehicles
Flipable vehicles
Lots more bicycles
Can siphon gas from
One tap of a tree or concrete block does not destroy your car and everything that you love
More row boats/boats
Fewer helicopters
Helicopters MUCH harder to fly/maintain
Can push vehicles
Exploding cars can kick it's contents out onto the ground (not instant loss)
Cars occasionally need to be jumped with jumper cables if the battery is almost dead
Shopping carts/trolleys (like those used in The Road)
Cars that are manual, rather than all automatic; would give you the ability to save gas and be a little more silent (higher gears at lower speeds)
If you happen to come across a police car (if it's implemented) it automatically has a radio built into the vehicle.....you can now scan the freq's for survivors
If ambulances are in.....^ ditto
Repair shops are the only places you can do engine changes (might mean you and a fellow survivor or two must push the car towards the mechanics garage
Engines have a far more advanced repairing mechanic (tool belts, spark plugs, hoses, etc) in addition to total engine replacements
Ability to partially prevent bullet penetration by using junk loot to block paths (stuff phone books, etc)
Ability to torch a vehicle with gas/matches and push vehicle down hill (or something) and cause a distraction
Offensive modifications (snow plows, barbed wire, scrap metal spikes, etc)
Functional trunks
Operational windows
Ability to camouflage parked cars (with branches, camo netting, grass) so that hiding your car is more subtle than currently. Perhaps make it so the car takes damage if you try to drive off without removing the camo parts.
Ability to upgrade important components with very rare loot, so that survivors can work towards a superior performance vehicle (eg. Offroad tyres, exhaust silencer/muffler, turbocharger, nitrous injection, larger fuel tank)
Ability to upgrade the standard locking system of a car - so breaking in and hotwiring it takes longer
Different levels of headlight power (eg off, 1/4, 1/2, 3/4, full), so you can creep along a road on a dark night with the headlights at 1/4 power so that you are more difficult to spot communal vehicle radio, so that all passengers in the car are forced to listen to what the driver wants ability to do realistic things while in a vehicle like drinking, eating, combining ammunition, reloading weapons
DIY LoJack, get two GPS units and some tape, follow someones car back to their base using existing tech
Duck! - vehicle occupants can lower themselves into a brace position (so they are less exposed to fire) but are forced locked into first person perspective while ducking (looking at the floor of the car)