Default vehicles tend to show off a specific thing. Jeep is a wheeled vehicle, ball is a physics no seat vehicle, horse is a bot, tank is an invisible wheeled vehicle with a bot mounted on it for firing projectiles, tank turret is a bot that can fire projectiles, rowboat is a bot that floats on water and only moves on water, flying jeep is a wheeled flying vehicle, magic carpet is a wheeless flying vehicle. I'd say if these are made default they would have to introduce something new.
So based on my above statement, ideas:
Bus would allow for more than the maximum(?) seats?
That jet car thing, maybe more than the maximum to speed or something?
That van... being able to move around while seated in the vehicle?