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Well, the animation for the volley fire could be taken from archers, which involves shooting in an arc.
Or, could arrows be made like spells? They would be summoned as abilities from a spellbook, such as a poisonous arrow, a freezing arrow (shot + slowing effect + freezing effect), a fire arrow (applies a fire mark and maybe something like pyrokinesis), an explosive arrow (like a bomb), and some kind of siege arrow that would hit walls (like how a cyclops attacks, but from a distance).
Of course, not all of these arrows need to be included; perhaps one could be the main attack, and the other could be an alternative, but I think it would be good if the refugee elves in the free cities enchanted the ballista arrows. It could deal high burst damage, but reduce initiative, or have medium initiative but reduce damage.