i love jmoos ideas, and i was kinda thinking on the same line, so i may add my thoughts to them 🙂

-be able to fire a diamond BLOCK out of a cannon that has a 50% chance to break any block that is in its blast radius (that can be repaired after siege) and that doesnt have an inventory (chests, hoppers, dispensers, etc.)

EDIT: we would have to work something out so that redstone machines wouldnt get killed due to ^^, like be unable to kill a block that has a redstone piece on it, or just have the redstone suspended and unbreakable in the siege, and when the blocks get repaired after siege, the blocks that had the redstone on top of them would regen and not interfere with the redstone itself.

-i also love the crux idea, if u werent able to kill someone in the siege, and they just hide in a box or something, u can kill a crux instead. i saw this done on one server that used emerald blocks as the cruxes and they had a health bar that was depleted by repeatedly breaking the block. when the crux is destroyed the sieger would win and gain access to the claim as if the defender was killed.

those are my additions to jmoos great ideas 🙂