Thanks for the heads up @Florian. I've discussed this with Cerv, and yes he suggested using structure templates and/or Gooey's quest instead using cellular automaton algo for the maze world generation which will be wayyy tricky.Gooey's quests goes very much into the direction of what you propose:
It is not a world generator but takes instead the approach to add dungeons and other structures (dwarf halls) to an existing world. To give the player control over this a character called gooey will ask you for permission before a dungeon or other structure gets added. Those dungeons could also theoretically become infinite large but since there is no size regulating component yet the dungeons have just a bigger chance to be smaller.