Old Stratholme would seem like the obvious choice to me, in that it’s a) significantly changed from vanilla, b) kind of an awkwardly long rambling dungeon that I don’t think they’d want to bring back in timewalking or otherwise use for “real content” so it might as well get a neat collector mode like Old Scholomance, and c) people like it. (Somehow I don’t think there’s as much call for, e.g., old-school Razorfen.)