I'd love to see the in-game ARFF expansion have more vehicles, as opposed to a general ARFF.
- Crash Tenders [both in 4x4 / 6x6 / 8x8 chasis that hold varying amounts of foam / chemical retardant]
- Dragons [These are Tenders with an extended boom nozzle for fighting fires]
- RIVs [first attack, rapid intervention vehicles]
- Chemical Tanker
In terms of mechanics, The FAA requires airports to have various numbers equipment standards based on the length of aircraft that the airport services. These include both chemical agents and foam agents. Both of these could have gallon requirements like the new forestry update has. This would add to the realism and make the game feel more interesting in this regard.
Feel free to help refine this idea. I'm not a fire fighter, just a nerd :).