this could be turned into a quest-like feature
once a week you would be required by some NPC to complete a series of tasks (cautch some specific pokemon, breed something, gain money, find some item??)
once you complete the tasks you are rewarded a special pass to the event place (random hoenn island??), this pass is a single use item ofc
in the event place, for a short amount of time (30min??) you could encounter event pokemon (hordes??) and pick up special items (the ones you listed??)
or instead of the pass stuff, by completing the tasks you would re-spawn the treasure island items etc