I'm sure someone has probably made some of these already. But, here are the mods I'm suggesting for the humble first vehicle, the bicycle.
Lights
Bicycle light: Could be made out of the flashlight and some duct tape for lighting up what's in front of you.
Off-road bicycle light: Made with 4 flashlights and some duct tape for even more lighting!
Storage
Cloth storage: Made with cloth and whatever can be made to stick it to the bicycle. Adds x amount of storage.
Shopping basket storage: Made from crafting or harvesting a shopping basket. Double the storage! Limited to two stacks.
Shopping cart storage: Made from harvesting a shopping cart and other materials. Just imagine that you've somehow managed to jury-rig it to the bicycle. Triple the storage, cannot be stacked, for obvious reasons.
Frames
BMX frame: For when you want to do sick jumps! Increases bunny hopping height when pressing "C"
Cruiser: The seat is so relaxing that now you use less stamina when pressing the sprint key.
Mountain bike: Increased traction on soil, sand, rough terrain in general.
Racing/ Touring: Passive speed increase.
Cargo: Basically the storage mods above but instead in one package. Cannot be used with other storage mods.
Motor?
E-Bike conversion: No longer use stamina to sprint on the bicycle! Only downside? Has to be recharged with a battery.