You can create burning grass-
I've done it. Of course, it doesn't hurt men or cause fear, but it does make for some cool effects and low visibility. All you have to do is mod the stuff in the models folder- I copied the shrub/grass parts, renamed them wheat, and made one part very easily burnable, the other part with the same qualities as normal grass in the game. The reason you have to do a mix of unburnable and burnable is because to make it burnable, you have to turn it into a more permanent model and then units can't walk through it. I'll post some pics and better instructions some time. It can be set on fire by flaming arrows/torches, but the best is by a catapult- the bouncing projectile can set many on fire at once.
Sorry to be rather confusing.
Justin