I had, or I guess still have, an idea for Yet Another Web Framework. I've written four versions of it in three languages already. I kept running into a problem: if it was complex enough to realize my full vision, then I wouldn't have time to maintain it, especially if it got any traction at all. Plus, for each single thing I wanted it to do, there were pretty well established things you could use instead.

I enjoyed programming it as a learning experience and to scratch my own itch. But I eventually decided it would actually be harder to maintain the system than to write a dumb little web app for any given project. And that there were probably much more valuable things I could teach myself than My Excentric Web Framework.

On another front, while I have never written any code for it, I keep thinking it would be cool to have GeoCities back but updated for all the cool stupid things you can do in the modern Web. A place for your weird taste in CSS, in public, with no shame and open to all. But I keep wishing somebody else would do it, which is not exactly the thing you asked about but somewhat close.