This idea was proposed long ago but never done. I suggest a special world for Fortress-based PvP battles. Such a world would have a way to initiate a war. During a war, all blocks would be breakable by anyone and TNT would be enabled. After the war, the world would be returned to its previous state. Consider banning or limiting obsidian blocks. Block placement would be severely limited, but some basic blocks (torches, ladders, dirt) should be place-able to facilitate movement.
