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Honestly the more I think about it the more I realize it would be a bad thing. Why? Because there would be too many dungeons to do, with too many items and too many (broken) mechanics. But mostly just too many keys you'd wanna throw away, because you know every expansion has its fair share of dungeons everybody hated.
I'll give you one example: Stormstout Brewery. I hated this dungeon so much back in the day, it is the worst, most disgusting nonsense ever made in the history of this game as far as I'm concerned. If I got a key for that dungeon I'd immediately want to delete it.