I always thought that the cars needed the thing you put on the roof or on the side that is used to put a bike on it. It would be useful if you had a mobile base, you could just deploy the bike for risky scouting operations.
The idea: You drive your bike onto a square next to your car that has the bike carrier. You enter the car and open the menu for headlights and cruise control and you choose the option “Attach vehicle”. The bicycle is now attached to the car and will move along it as it drives. If you want to remove it, you open the options and choose “Detach vehicle”. The bicycle can now be used and moved again.
Problem: You could attach any other vehicle to it.
Solution: Add a weight limit. You could only attach vehicles the weight of a bike or a bit more for modified bicycles. If you attach anything heavier, your vehicle won’t move. Or maybe it could move, but very slowly, like 2km/h (sorry I dont know mph). It would be useful for removing ruined cars or car chassises out of the way.
Problem: Programming, obviously.
Solution: I don’t know shit about programming, so if this idea is impossible, just ignore it.