It's an interesting concept - to a certain extent, a similar game was put out by the same folks that made the SimCity (I think) series, and it revolved around EMTs....
The problem with a "Fire Truck" sim is not so much the modelling of the trucks but what to do with it. The question is - "Will a Fire Department Simulation Sell"
I think there's a hardcore of gamers that would take on the challenge - of which driving the truck(s) is the "easy" part. Make driving the truck the key part of a simulation and it's another "racing" game.... But an interesting.... one to say the least.
1. Realistic performance on the trucks - (Do the engines/pumpers/ladder trucks, emergency trucks handle that differently? You'd have different sizes for diffferent environments.
2. "Full realism action" Not all trucks have automatic transmissions, simulate loss of air brakes, etc. etc.
3. You come across a sudden detour in the road....
4. Urban/Suburban/Rural/ Forest Scenarios (take your Hummer fire fighter into the forest.....)
The missions are scored by how fast you can get to the randomly generated address...
Make an open architecture so you can have add-on vehicles and scenery ...
Now - where can you get decent scale drawings of the fire fighting equipment?