I tend to think of a lot of things I'd like to write, but never get around to doing. I have started a website to let people combine such vapourware together (in the hope someone will do something), it's called halfcoded.net. Unfortunately at the moment that's vapourware, too. Here's a few other things I haven't done.
"Theme" would have been a program to allow you to author a 'theme' for an abstract window manager, and then convert themes written for this abstract window manager into the configuration syntax of concrete WMs, using plugins.
I wanted to make a really high-resolution mandelbrot fractal poster whilst at Uni. To generate it, I thought I'd write a program. The plan was that I could make it output pure postscript, directly to the A0 plotters.
a window manager
I started working on a window manager called 'slate' (although I might have changed the name in light of the web magazine. I didn't finish it, although nothing I've seen since has done quite what I had in mind (awesome gets close).