A first person perspective game where the protagonist rather abruptly realises that the world they're living in isn't real. Everything is a fake amalgamation of their memories, or an incongruous lie that their subconscious can't accept. The world they're in is some sort of illusion, and they have to try and find their way out of their own mind and confront whoever or whatever has trapped them in there, and find out why they've been imprisoned in their mind in the first place.
The only problem is, someone else is out there. Some other intangible force seems to be controlling the unreal world in which they exist. Some other power is altering aspects of the world at will, in an effort to prevent their escape, giving the protagonist little to no control over their own mind. All the protagonist wants is answers, but, at times, it seems like there's no way of knowing if the clues they come across are hinting at the real story of how they ended up there, or just lies to placate them.
The protagonist's journey will take them through their memories, but it also goes out of their own mind and into the minds of others (or, at least, what they hope are the same subconscious illusions of other people stuck in the same situation) in an effort to wake them up and rescue them too. But how can they know who is real and who isn't? Along the way, the protagonist confronts their own repressed fears, memories and insecurities, given physical form, as they try to figure out the aspects of his/her life that she/he can't remember. Every time they pass through a memory, it raises more questions as they try to distinguish between the illusion and reality.
Enemies will also need to be confronted along the way, but not through violence so much as using intelligence, platforming and the broken rules of reality to deceive/avoid/kill them before they kill you. The world doesn't operate according to conventional rules, you see. Levels can suddenly and unexpectedly rearrange themselves, resulting in different configurations. Walls may be glitched, or gravity may abruptly invert half way through the level, leaving you on the ceiling. Even light or sound may not operate the way they do in the real world, in certain places.
The best way I could summarise this is Psychonauts crossed with Silent Hill crossed with Portal crossed with Abe's Oddysey/Abe's Exodus. I would just love to write a story driven game designed around the concept of sheer mindfuckery, that plays differently from pretty much anything out there on the market right now. Ideally, the game would also get under players' skins and really creep them out, making them wonder if something in the game is watching them, toying with them.
In Soviet Russia, game plays you?
(Long post is long. Sorry.)