Bicycle: a “gear” mod to make it go faster, but start/stop/climb slower. Like a 10 speed but with only the 10th gear. And then another “gear” for climbing/fast start/stop but no speed, like a 10
apeed but the first gear. Then if you put them both in you have a functioning 10 speed. Slow/climb and fast top
end. Then nerf the normal bike a little for top speed.
gyrocopter: it’s kinda been mentioned above, as “seat belt”, but I wrote a long time ago it would be nice to have a “safety mechanism” of some type for the gyro. .... even though it’s perfect to be able to just jump out

. I vote for a “rope” or something you tie to yourself so when you do fall out you dangle below it and it slowly crashes to the ground with you. Same for other vehicles even though “seat belt” is probably better: you fall out and the vehicle stops going forward (it decelerates naturally and just drags you along behind it

maybe you live through it, maybe not. I do favor the “eject” features mentioned above though when hitting/flipping a vehicle.
All vehicles: add the ability to put a “basket” or chest or a seat for any “open seats” on the vehicle. Meaning: you have to choose to add a “seat” or “storage” to either carry people or stuff. Then for single player you will be able to add more storage (up to however many seats there are) and for multiplayer you can mix/match based on what you need.
All vehicles: when adding “engine enhancement “ mods (speed/turbocharger/etc) change the engine sounds.
all vehicles: make the “sounds” (e.g engine or pedaling loudness) of the engine attract zeds, then put muffler mods on to deaden the sounds so they are less attracted you it.
all vehicles with engines: make them “handle” worse by default (slip and slide, turning radius bad, etc) and then add mods to make this better (suspension, steering, etc).
all vehicles: “armor” or “welding” or something mods. Purpose: to increase vehicle health. Then nerf the hell out of them so they have low health by default. Make traveling with no or little armor/welds? Kinda dangerous and make you stay on roads. Currently every vehicle feels like an unstoppable tank right now, and repairing (a single repair kit?) to full health is way too easy. I’m not aaying a 4x4 should have 10 health by default, but make it take more serious damage or something when you jump/fall/hit stuff.
the whole point of all of these is also to not be able to mod the hell out of a vehicle to “max it out with all available mods” but to have enough mods for each to force you to choose how to mod it, and make the vehicles kinda bad enough that you DO want to mod it. E.g “I have 4 mod slots but there are 10 mods to choose from”. This also would give some need to craft a few of each vehicle (a traveling car, a storage car, etc) and extend the game a choices a bit.