1. Make a chest spawn randomly in the world. Put good stuff in it, unique items to start a quest, items to collect to make an awesome weapon/shield/trinket, tons of gold and maybe some other stuff like enchant scrolls. Also when first spawned it can't be looted for 30, 1hr, some time limit
2. Make a world announcement or city wide announcement when chest is first spawned
Now watch the pvp ensue.