Allow catapult rock munitions to be made out of boulders from the quarry. Allow necromantic corpses to be made into catapult ammo corpses. A number of caltrops could make shrapnel ammo. Not sure which skills to use, but it'd be nice to see people using catapults in pvp. Because certain hilarity will ensue.
It'd certainly be a nice addition for those who seek to carve out a piece of territory for themselves, either temporarily or permanently. Bandit ambushes, Society Lowerdark camps, dwarven holds, etc.
Considering how long it takes to set up a catapult or crossbow, I can see it adding more to PvP beyond buff and crush, which most PvP is in the end (not to say the RP isn't good usually). When one has to take into account an enemy's defenses, be they traps or someone manning a catapult, things can get a lot more interesting.