As the title says I like the idea of having ballistas.
They shouldn’t do structure damage. We have trebuchets for that. (movable catapults would be a great idea)
They should do a lot of damage against infantry. 100% armor penetration. Damage depends on ammo type.
They could also be another strategy against avatars. Arrows do 1s, Tribuchets do 20s, Ballistas could do like 10s of “damage”.