I came up with an idea for a video game 30 years ago: it would be a first-person-viewpoint 3D graphics game about walking fast through a crowd, trying not to bump into anyone.
I've always felt that the tech wasn't quite there yet to implement it right, though VR might change that. (The specific thing I think would be difficult to implement is the sense of proprioception, giving the player a sense of the character's body as it moves through a space that's largely filled with other people who are moving in roughly the way that real-life human crowds move in a public space like a sidewalk or a train station.)
Today, while walking through a crowd, I thought of that game (as I often do), and had an epiphany:
You could implement about 90% of the core idea as a top-down 2D view of a square navigating through a crowd of other squares, with no need for any of the complicated 3D graphics or collision detection that I've been assuming all this time would be necessary.
Would still need a crowd-behavior system, but I bet some flocking algorithms could do a good enough job on that part.
(See also Facebook thread for this post.)