There's nothing stopping me from 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 for it. If there were an animation, I'd have to abandon the idea of Dwarves because I can't synchronize their movement with the ballista's movement.
I don't want to make a mortar; that's not in the style of Ashan:D. But I could give one of the upgrades a close-range flamethrower-like weapon (like Greek fire), and something else for long-range, like a magical stone that shoots darkness or fire.
Added 2 minutes later
To clarify: I mean making the movement with a teleportation animation
И пусть Эльрат осветит твой путь