Other levels of Chief is effectively implementing a command structure into the game - which I would love to see. It would mean that the more advanced a call, the higher the command level needs to be on scene.
I had the idea that instead of having 5 different vehicles for five levels of command (Or any other number depending on your preferance) You could have a "Supervisors vehicle" which would be crewed by staff with training to a certain command level.
For example, In the UK we have Firefighters, Crew Managers, Watch Managers, Station Managers etc... It would be nice to be able to assign one crew manager per fire fighting vehicle (Engine / Water Carrier / Ladder etc) and then be able to assign a watch manager to one vehicle for larger incidents and have a specialist Supervisor vehicle for a Station Manager for even larger missions etc..