Naxxramas and Blackrock Mountain are based on some of the largest high-end areas in WoW - Naxxramas was the final raid of vanilla content and remains one of the largest. Blackrock Mountain is an enormous dungeon and raid hub in Azeroth - containing not only a large un-instanced interior but also 4 distinct dungeons and 3 raids.
I can't think of many other comparable places in WoW, though winged raids like Icecrown Citadel, Throne of Thunder, and Siege of Orgrimmar. But Adventures could also be drawn from expansion themes (a Burning Crusade adventure) or even PvP (an adventure based on classic battlegrounds).
Personally, I think an Ulduar Adventure could be fun. Anyone else have any ideas for Adventures they think they'd like to see?