District Chief - Other than just renaming, Maybe we could expand the Command Staff vehicles for a Chain Of Command type of thing. Like if a call is larger where you need 2 or more Battalion Chief Units then a District Chief can be present to oversee the situation.
Same basis as an EMS Chief responding to 10 or more patients but instead a District Chief being required when a certain amount of Battalion Chiefs are responding.
Canteen Unit - Someone has already mentioned this but didn't get much feedback so I figured I would bring this up a well. Not everyone has this so this could be an optional unit to give more credits to a call. This would be mainly for major incidents that would generally call for one of these units.
Transport Unit - For Fire and Police as well. Besides the Utility Unit to do this perhaps a designated unit to transport Firefighters and or Police Officers for extra manpower.
Trailers - Any in General like Hazmat Trailer, EMS Trailer, Kiddie Foam Tender Trailer etc