RTF is a global modification that replaces all towns in the game.

Nothing prevents making a tier 4 unit with a low stat increase.
The problem with the ballista is that it can't move at all; there's no animation. If there were an animation, I would have to abandon the idea of gnomes 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. But one of the upgrades could have a close-range flamethrower (like Greek fire), and for long-range, something else, like a magical stone that shoots darkness or fire.