Something like a Hand Cannone could be fun. Very primitive, handheld cannon. Heavy and clumsy, innacurate…but powerful. It could be given bonus damage against objectives like the doors of Citadel, or other seige weapons. Maybe do partial damage to things like the Palisade wall of the same map, that normally needs ballistae to damage.
Balance out the possible objective cheesing by limiting the ammunition heavily, and making the weapon reload painfully slowly. Might encourage more melee archers.
This would be to the archer class right? or are you suggesting that a melee class would get something this powerful and ranged? Imo anything that oneshots this easy would feel cheap and stupid.