What about a dwarven flamethrower? Random thought it could produce a fireball type unit that would do collateral dmg, slightly more dmg than a fireball, would not be able to bombard cities. When the unit attacks directly it would not do collateral dmg but would do higher attack. I also thought about the idea that is could destroy ancient forests but could not spread (unique niche imho). I have no idea if this would fly but it struck me as interesting for the khazad.