Firstly I want to say that I love the game but I want to throw some suggestions for additional units.
Rescue boat: boats for swift water or dive rescue situations for calls such as person drowning, vehicle in water, sinking ship or ferry, occupied home flooded, and so on.
Fire boat: for fires along or in a body of water. For calls like boat on fire or a fire along a waterfront. Maybe a call like pier on fire.
Foam units or a chemical engine: industrial fire fighting is full of them because foam foam and more foam is what's effective for fuel fires. Can be used for plenty of existing calls like unknown tanker spill, industrial fire, gas station fire plane crash (since they do supplement ARFF apparatus in some departments)
Brush truck/type 6 engine: a small apparatus crewed by 2-3 usually mounted on truck frames used to go off road to fight grass/brush/ forest fires. Many departments use these and they can be used for many of the smaller fires in game that they are made to handle. It should be in a desperate category from engine to distinguish it while dispatching.
EMS Supervisor: acts like a fire chief for EMS calls with multiple patients. The fly car can be used as one for now but it acts more like a non transport EMS unit than a supervisor.
Police supervisor: who doesn't love the salty police sergeant who greets everyone with a grunt? The police supervisor will act as a patrol- supervisor on calls since they can do the grunt work as well as command the scene as more units arrive.
More space on the platform trucks would be a big help too since in the department I worked for there was enough space for 6.
