Ballista would be cool! Wouldn't mind seeing rock throwing and boiling oil too, for defense. Give players options, instead of relying on AI Guards.
Need more options for attackers too, like a siege camp which could act like an Inn and minor storage for boulders etc Siege towers, manganons with different ammo types(Rock, Explosive, maybe carcass?).
Was thinking about how ninja sieges occurred a lot in MO1, and a way to lessen that would be to only allow siege equipment to be built if within a siege camp radius. Adds more risk v reward. Gives more time to be discovered and allows defenders more time to muster up.