OOOH MAN THIS SOUNDS EXCELLENT. I was actually thinking about something like that, except it actually being in the castle itself. This seems a bit less complicated to pull of, though :V

With Frisk, it could range from fun--going through the Underground without killing anyone, especially moments with Undyne or Papyrus--to calm/sad/lonely-feeling times in their home city, to the horrible nightmares that were Photoshop Flowey and the True Lab amalgamates. Or their friends trying to kill them.