I'll skip right to the point I seriously dislike that other than a handful of chests they are all one time only. I enjoy treasure hunting and occasionally finding myself in the vicinity of a rare mob or rare hunting and finding a random chest. The chest locations aren't dynamic, once you've done them once the challenge behind finding them greatly diminishes.
Some Suggestions:
After finding all chests you essentially phase into a competitive stage where random chests spawn but it's first come first serve. PvP servers could even universally flag someone while they are opening a chest leaving them vulnerable to same & opposite faction assaults. If a player hasn't been assaulted by the time the chest is open then the player becomes unflagged and only has to worry about opposite faction assaults. Pve servers could have a 1 minute timer start on a chest after it has been looted before it vanishes giving others a chance to loot it, perhaps with a loot limit.
Rare Mobs drop Keys - The Zone could have included a variety of rare chests in a large variety of locations only unlock-able after getting the appropriate key. This could either be first come first serve or individually phased.
Daily Hidden item quests - along your travels throughout the island you could find npc's looking for specific rare items that can be found either from random drop chance or physically finding their spawn location.(That one npc kind of does this but in my experience I generally come across the items without even meaning too)
(PvP Only)Bounty Hunter Repeatable bounty hunter quest where you are instructed to hunt down and kill specific players on the timeless isle(if the player leaves, the quest will change dynamically to instruct you to hunt down someone actively on TI. The more people you hunt the more likely you will be chosen as a target for others!!).
(PvP Only, maybe PvE too) Rare Mob Allegiance - You can choose one Rare mob on the entire island to pledge allegiance to and whenever you enter the vicinity of that chosen rare mob you will become allied with it and become hostile against all of those who engage them. You will be able to loot other players you help defeat!!
Let me know what you think of my ideas and I encourage you to talk about your own!!!