The good thing with Caverns of Time is we already have a portal to it from the capitals, it already features plenty of events and it also was a fairly ambitious area for Vanilla that was fairly poorly executed. A modern version of the Caverns of Time would host all the dungeon and raid portals, the anniversary event, could have all the timewalking vendors plus this new game mode. And yeah it could use areas and mobs from every expac ever. We could come up with dozens of interesting scenarios for which we'd already have the area and the relevant mobs in game.
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I know all villains in WoW have been dying in raids so far but . . . why? We know 10.1.7 will have storyline advancement. Why not make the story be about killing Fyrakk?