I like that idea, I think custom dungeons would be a very popular feature like custom maps in WarCraft and StarCraft. It would let the community solve the content drought problem for them. The WoW engine doesn't allow for content creation in game, but they can have an external Blizzard Dungeon Creator, similar to the map editors of the previous games and let you upload those custom dungeons and then play them through the game.
We all know the first dungeon to ever come out would be shaped like a penis
This world don't give us nothing. It be our lot to suffer... and our duty to fight back.
After playing new world I can safely say that what WoW is missing isn't sandbox. Sandbox = no content. "Just make your own content 4Head" is a pipe dream that always falls on its face.