Had a handful of ideas between myself and a friend of mine and thought it’d be worth the time to type it up. Up to you folks to decide if it’s worth the time to read it 
ANYWAY…
Vehicle Plow
Something akin to a snow-plow, But I doubt any of us think ‘snow’ in this case. Useable for pushing vehicles/etc out of the way of smaller size (Weight and actual size?) with the damage only being transferred to the plow rather than the rest of the vehicle. (So very tired of losing ten ton vehicles because I clipped a motorcycle…)
Random Loot in vehicles
Not a lot, just some stuff, gloves, napkins, the random crap people keep in their cars? Military-Themed junk in the Military Transports, School crap in school buses, and so on. Not asking for hordes akin to survivor RVs (though that might make for an interesting rare event) just /some/ loot in the not-so-destroyed vehicles.
Interior Radio/MP3 Player
Installed in the same fashion as horns maybe?
Wall-Mounted Containers
Because you can never have enough stuff.
Mine Flails
Not necessarily for ka-booming mines before you run them over but more for smashing zeds down and dead before they bounce off (And as a result break) your windshield.
Treads
…What it says on the tin, I just think there are occasions when traditional wheels have failed me. Plus a Half-Track might be interesting to have…
Firing slots
Something high enough that it doesn’t provide much a view when “open” but keeps the zeds out and allows you to fire out but allows projectiles to pass though (Both ways)
Fire Suppression Gear
Akin to water-pumps on fire-trucks, only practical for vehicles that have forward-mounted flamethrower turrets.
Mounted Rainwater catcher & purification
Since the water tanks you can install only take “clean” water the idea here would be something like the funnels except mounted on the vehicle, which is then (at an energy cost) turned into water for the tanks. Not sure how practical but it’s an idea.
Hydroponics
Again, not sure how practical but something of a mobile farm… Sitting still in the game makes me nervous. I like staying on the roads…
Amphibious Gear
Probably a snorkel per engine, a propeller, and some stuff to make it float. Probably impractical for larger vehicles.
Jury-Rigging
For when you’ve got duct-tape but not the proper parts. The “repaired” device has a chance of catastrophically (Engines explode, tires fall off and cause skids at speed, leaking liquid tanks, etc) failing but it’ll run for now.
Vehicle-Towing/Tow Points
I’d love to see Semis towing their trailers but what the main idea I had here was building and then driving Dead Reckoning though assorted towns.
There’s what I got, what do you folks think? Not sure how practical any of these are but it’s a thought.