While it would be realistic, there are still the 2 unrealistic things that it starts burning because of swords, pikes etc. and that you can extinguish the fire with a hammer.Here's a thought - if an unit is attacking a building and the building catches fire the unit should get some damage, you know, because of the fire. Subsequently, once it is on fire, it should start damaging the building itself slowly the fire should increase. So a low fire can start decreasing building hp by 1 per sec.
I think that would make the game very interesting..