Well, I have quite a few ideas but I'm afraid someone who actually knows how to program would steal them. I guess I could give one of my worse ideas, though.
At the beginning of the game you are told that you are a human museum worker who has lost a priceless artifact when you were robbed by a bandit, who you couldn't actually see at the time, on the way to deliver it to the Smithsonian.
You are tasked with getting it back, however, shortly after this, it begins raining knives, quite literally. So you head into the Smithsonian only to find that the entire place has completely changed. The exhibits have all disappeared; in fact you can't find the entrance anymore, instead a strange world has formed around you.
The game-play would be part RPG, part adventure game, and part psychological mind fuck. It would mostly consist of running around exploring a massive world that at times will make you think you are on acid, fighting against enemies and leveling up you long list of skills.
The game would present you with hundreds of strange and unbelievable situations, and (sort of in the vane of Silent Hill: Shattered Memories, except much MUCH deeper) judge the players reaction to each one, altering further imagery in subtle ways to best mess with the player: If he or she shows a strong dislike of spiders, then spiders will become frequent enemies, etc.
A few of the things that would be encountered in the course of the game:
At one point you come across a flower. If you attempt to pick the flower, it turns out that there wasn't actually a flower there, but a bear trap, which grabs your arm. As you try to get your arm unstuck you see an ant hill under your left foot. ants crawl out, and, after freeing you from the bear trap, grow to about human size. They tell you that the bear trap is actually a beast that eats the memories of those caught in its trap. They implore you to help them defeat it.
You come across a torch which actually darkens the area around it, even though the flame itself is still normal looking, that is the only light it creates. If you walk around in some areas with it equipped, you will find that you can only see some objects, items, etc. with it out. In one case you find a series of platforms that allow you to reach a new area which is in perpetual night. In this area, the torch's affect inverts: it brightens like a normal torch, though the flame now is invisible. Instead of showing hidden objects, it hides visible ones.
You find the bandit who stole the item. He is a ten year old child. He is a boss fight towards the end of the game, and depending how the game categorizes you the boy may play a larger or smaller role in the game.
Eventually you come to the final quest, at the end of which You enter a small house. In that house is me, the developer. I explains that this (the world you've been exploring) is actually my brain, and that the main character is a simple figment of my imagination. Shortly after their conversation ends, the player character walks out of the cabin to find himself in the Smithsonian again. he looks around, walks a few steps and then disappears.
The credits roll, as you see the Smithsonian warping and changing into various different environments. In the end, all that is left over is a blank white slate. A message displays that thanks you for playing.
Note: If this idea seems out there, that's because I was coming up with most of it off the top of my head. If I were actually to make this game, a lot of the content would change, based on what I was thinking at the time of development (It is a portrait of my mind after all.)
The art would look semi-hand drawn, and would be somewhat Picasso inspired.
tl;dr: It's a game set inside of my brain.