(1) Muffler - Make the 4x4 quieter.
(2) Sleeping bag mod ... You (the owner) would respawn at the vehicle.
(3) I'd like to take off the spikes/ram plates OFF the default vehicles and make them mods. In this way, the default motorcycle and jeep would do better off-road and you could choose to sacrifice mobility for doing increased damage when running over a Z.
(4) I'd like to be able to choose between storage space and passengers. So, on a motorcycle, the player can either add storage mod or add a passenger mod. On the 4x4, you could either add 2 storage -or- 2 passenger -or- 1 storage and 1 passenger.
(5) Perhaps a light module, that adds extra lighting -- wider, further, brighter. This can be handy for those who work at night an illuminate their projects with the headlights of their parked motorcycle/4x4.
(6) I've seen players ask if there was a way to have a workbench in a 4x4.
(7) I could see a trailer hitch mod that lets you tow a specialty trailer around. The trailer could then have things like a workbench, cement mixer, chemistry station, more storage, etc.
This isn't a mod, but I think it would be cool to have an electric golf cart in the game that we could recharge from solar panels / gas generator / battery bank. It would make for a nice "get around town" vehicle. You could fill it with batteries from the game to determine its capacity.