Just a note, the bowling ball catapult would ned lead to cave-ins since these would be a result of the actual arc.
setting targets for catapults is really important, and would for instance allow for having multiple catapults with multiple siege dwarves raining stone into a confined area, such as a valley leading to you fortress.
More importantly is that siegers should set up catapults and bombard your defences. is they had a chance (say 5%) to break a wall or other construction, and often cause cave ins of roofs, collapsing bridges etc I at least would build my fort differently, and I would enjoy the added task of building something which can withstand an actual bombardment.
This would add so much flavour to the game it's simply ridiculous. having built a keep several walls thick to withstand fire, and manned in smaler fortified compartments with marksdwarves while stones rain down onto the castle, eventually breaching a fortification causing death and injury at the ramparts. Then the goblins attack, using ladders they aim to enter at the breach and i evacuate the marksdwarves and send axedwarves to hold the ground against the golbin horde.
Meanwhile the stones keep on raining down on other sides of the fort, causing the bridge to collapse. But the dwarven catapults have finally started to aim in on the goblin siege engines and with a few well aimed shots the menace is taken care of. The Staunch axedwarves hold their ground and the masons effectivly repair the fort closing all the breaches and gaps. Seeing their previous advantage gone the golbins have to decice wheter to flee or continue their attack by some other means. In a moment of bravery they decide to storm the gates. They run around the castle and place a ballista in front of the main gates of the keep, blasting the heavy gates to smithereens with their iron ballista bolts they place a ladder over the moat and storm into the keep.
The marksdwarves are now in a new position and quickly kill off the goblins in a hail of steel bolts.