First! Ballista is must have for dwarfs becouse we need defend against dragons that wants our booty!!!
Dragons is must have too! Dragon kids for normal mobs, normal dragon and Adult huge dragon who is titan!
Dragons will atack from air and after few atacks they will land so even warriors can atack them, but after some time they will fly again for short time and casts fireballs! And for the adult dragon and titan, few ballists will be must have!

and maybe add this too. (this was first message xd but this is not that imporant)
Maybe you can add something like new type of life in the game? New biomes like Magic forest or something like that and very hard to play biome ''Top of the mountain biome'' Or just add new traits? There is only few traits in this game :/ And maybe add new items for enginers like ballista with very slow fire rate but huge ammount of single target damage and very expensive ammo? Or catapult that needs huge ammount of time to load, low single target damage but huge aoe damage and can aim great distanse even the other side of the mountain. And for dwarfs you need to add new UI inside mountains and huge dwarf doors like in Hobbit.