Suggestions for vehicle enhancements...
Altimeter/gyroscope for gyrocopter. So I can see how high above the ground I am and if I am climbing or descending.
Speedometer (should be self-explanatory)
AC and Heater mods. So that I can be comfortable in any biome without the need for armor/clothing mods. (At least until I get out of the vehicle.)
Flood lights... High Intensity. So that I can see farther at night. (including the ability to put them on a gyrocopter)
Night Vision? After all, Cadillac has been doing it for almost 20 years now. (though admittedly, it will probably be the same exact effect as wearing night vision googles now while driving.)
GPS. Not so much for turn by turn directions (although that would be nice, it is probably overkill.) But, just a corner of the screen that can show the overhead map of where you are located so that I know when I am close to my turn.
Rotating Blades. Don't just knock down zombies that you hit... Maim, dismember, and kill them.
Two other vehicle ideas. (possibly as mods)
Nitrous Oxide. Create from nitrates in a chem station. Increase vehicle speed.
Passenger Turret. Either a hole in the roof for 360 degree aiming, or let them lean out the window and only aim on one side. Can be an actual turret with ammo, or the player's own weapons. (including a baseball bat for smashing mailboxes or zombie heads at high speed.)
New vehicle suggestions:
Skateboard - Faster than walking, slower than a bike, no inventory.
Sports Car - Faster than a motorcycle, uses as much gas as a 4x4, between the two in inventory space. 2 person max.
Sedan - roughly as fast as a motorcycle. uses less gas than a 4x4, more gas than a motorcycle. 4 person max. Between motorcycle and 4x4 in inventory space.
Pick Up Truck - Similar stats to a 4x4. More cargo space.
Electric Car - Uses an internal battery bank for fuel. Less range than a gas engine. Possible solar recharge mod, or you will need to swap batteries at your base. (similar to a Sedan in other stats.)
With the idea of turrets, maybe a APC or SWAT vehicle. Can have a player operated turret for MP, or an auto-turret for SP. Uses a lot of gas, but has a crapton of cargo space. Can carry 4 (or maybe more.) Possibly allow it to use an auto turret to stop Zombies from entering the POI after the player goes in (or give support fire to players that rip through an entrance with the stealthiness of the proverbial bull in a china shop.)