Dream dungeon?

OK, so -- the idea is that the characters get a way (the Helm of Subconscious Materialisation, I dunno) to project themselves into an unconscious character's mind so that they can retrieve some vital piece of information. Like, the only guy who knows a particular spell is in a coma, or the only one who knows the location of the missing princess, or what have you. So they have to go into his head to retrieve the knowledge, only the inside of his mind is essentially represented as a dungeon.

I guess that's The Cell, basically.

Good points:

• It fits the existing metaphors we have about the subconscious -- for instance, we tend to think of the subconscious mind as being "down" or "buried" already. 
• It solves the problem that not a whole lot in a dungeon may make sense by specifically using dream or psychological logic rather than physics. 
• Speaking of The Cell, go nuts with all kinds of wacky Tarsem Singh imagery. 
• But, crucially, it isn't totally random, and the players can still use their heads to figure out what they need to do; they just need to think thematically/emotionally. 
• You can fit all kinds of surprises and clues into it in the form of memories and so on. 
• Things the players do in the mind can actually affect the character's knowledge and behaviour when they wake up. 
• What if some of the subconscious bad stuff makes it out along with the PCs because no one really knows how this works, so now Uncle Timothy's id is out there just running around unrestrained or whatever. 

Warning signs: 

• Works best if it's an NPC that the characters know, so that it's not just "what is in the depths of a person's mind" but "what is in the depths of Uncle Timothy's mind?" Then you can both allow them to use what they know to figure out puzzles but also surprise them. 
• I guess you could get away with it if it's a character that has a lot of heft in the game world -- like, if you're going into the mind of Batman or Robin Hood or Merlin or something where the idea that they have certain traits is already well-established in the expectation of the players. 

Alternative version: 

• You could just have it such that there's some powerful force in the world that has an externalised mind in the form of a dungeon or a palace or something -- so Galactus is out there rampaging and the reason he can't be mind-controlled is that his space station is actually his mind, so you have to board it and get in there to reason with him or find out his weakness or whatever. You could either set this up such that nobody knows this and the players just go to the palace to rob it or the space station to desperately find a weapon to beat Galactus with, or you could have the fact that the palace controls the force/person's mind be a well-known thing so that it is jealously guarded by that person's faction and different groups are trying to take control of it. 

Designing as a product: 

• You could either approach this dungeon just as a plain old dungeon where the gimmick is that it's someone's unconscious mind orrrrrrr you could go full nutso and (this is much more difficult) turn it into a system for generating what a person's brain-dungeon looks like. So, to take a D&D 5th example you could say, well, what about a person represents the things you might find in their mind? Well, their class and race, obviously, levels, Background, Int, Wis, and Cha, right? So each of those things leads to or influences a set of random tables or lists of elements you use to create the dungeon. Maybe what level they are (ie how old, how experienced) determines how big the dungeon is, for instance? 
• Again, it depends on the system and setting assumptions. So, for instance, if we were doing this in Vampire: the Requiem, the house would be a spooky old house full of all the character's old enemies, dead friends and lovers in which horrible scenes from their past were constantly being played out and their Beast was a literal monster lurking in the basement. 
• Or in Glorantha it might be a big hall filled with their ancestors, but then those are games that already have a pretty good line on modelling the character's unconscious and social context, you know? I think that the interesting thing about this idea is that dungeons are such a good metaphor for the unconscious mind in a game where the focus on challenge and action is often thought of as detracting from engaging with that kind of thing. 

Anyhow, that is just a thought I had after watching cartoons while trying to go to sleep. I guess it is mainly just Labyrinth already, or maybe A Nightmare on Elm Street, but whatever.