In my opinion, creatures shouldn't become completely different after an upgrade. A ballista turning into a flamethrower is a bit much. Although, who knows what dwarves are capable of.
Added 4 minutes later
There's nothing stopping us from making a fourth tier with a low stat increase.
The problem with the ballista is that it can't move at all; there's no animation. And if there were an animation, we'd have to abandon the dwarves because I can't synchronize their movement with the ballista's movement.
I don't want to make a mortar; it's not in the style of Ashan:D
Блистает там воздушностью прогалин.
Но между кедров, полных тишиной,
Расщелина по склону ниспадала.
О, никогда под бледною луной
Так пышен не был тот уют лесной...