Let's be honest. Not every engine has a Class A Foam Tanker on it. Especially the older ones. And you can't use just water on a grease fire, Aircraft, Burning car, electrical equipment etc that has some kind of fuel or fluids. You need foam to assist you with it.
Either a foam tender or an optional upgrade to put on your Type 1 & 2 Engines, Truck, Rescue Engine, Quint, or Tanker.
While I'm at it how about different optional upgrades to add onto the units instead of keeping our trucks "Basic".
Like adding a "Light Tower" for certain engines, rescues or units that can have them on it.
Extended "Crew Cab" for engines or trucks that can house more than just 6 firefighters on them. I know of several trucks that house 7-10 Firefighters on them that includes "Jump Seats".
"Traffic". An option that the player can turn on and off. Traffic would cause the Units response time to either be slower or faster.
"Routes" Giving the player the option to decide how their fire, ems, and police units respond to the call which can also work along with the Traffic option to give the player that the sense of realness to the game.