I agree with the X times to get a key item for what you want, but it other than for a major event such as nyzul isle, I don't see it being done or even that viable of a way to go.
Major problem with this sort of system is if you make it a key item per lets say a voidwatch NM, that's more information stored per character from a good ~40 different mobs, it's more npcs, etc. The amount of stored data adds up very quickly when they're already keeping track of hundreds of key items, storage slips, multiple inventories, skills, cruor, CP, etc. etc. etc.