So I think that a few things can change with units and a few units could be added.
Changes:
1. Add a "Light Rescue" and "Heavy Rescue" category. "Light Rescues" could deal with minor situations such as minor MVAs and basic rescue calls and allow "Heavy Rescues" to deal with all major calls. An example of this would be utilities and ladder trucks would be in the "Light Rescue" Category and Rescues would be in the "Heavy Rescue" Category.
2. Make some calls (Such as gas odor) not require purely an engine, some calls that call for an engine could be dealt with by a Rescue or a Engine.
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Units to add
1. Safety Officer - The safety officer would be an option unit that would speed up the incident and allow it to be a requirement on Large Scale Missions.
2. Higher ranked Chiefs - Make Larger incidents require higher ranked chief officer.