Hi, i used to play a mmo which had a very fun concept of having a so called unique items(robots for example). These were very powerful (but not game-breaking) items and only one instance of them could exist in the whole game world. Usually obtainable through a mini-quest (these quests were positioned always at same well known world position) and guarded by a spawn of powerful npc-s. These items would decay over time (usually a 1-2 day period) and finally got destroyed after the timer expired. After they got destroyed there was a small random period (usually 1-2 hours) and the mini-quest would reset, making the item obtainable again.

This led to a very interesting fights over the uniques, and could improve the beta experience.