I found an old game called Elite from 1984 and thought the graphics were impressive for their time. I thought that it would be fun to make a multiplayer remake. As (reportedly) commonly happens with these sorts of projects, I put a lot of work into the engine, but never got around to implementing the game. I think in my case, a large part of the reason for that was that there was so much unexpected infrastructure I had to add: renderer (obviously), file system abstraction, console commands (not strictly required), key binding, scripting language, etc. Then I started duck-lisp and that took me away from the project for three years. Then I came back and decided I wanted to make an actual game, or at least a demo. So far, that's actually been working out. I've put a lot more work into the engine and added SQLite and instanced rendering, but those features are in direct service of the little demo I'm making.
Games:
Sandbox