When the repeating crossbow is shot through a torch or other flame source, the arrow should catch fire. This would then catch the things it hits on fire
They already have this. If you place a torch or flamethrower in front of the crossbow, only about 20% of the arrows actually catch fire. You have to burn the crossbow itself for 100% flaming arrows. However if you do not have a water canon ready your crossbow can only fire about 15 arrows before burning away