I've spent a considerable amount of my free time over the last few years working on an open-source game engine for making Zelda-like games (link in profile). It's been around for a few decades, and I played it when I was a kid - and now I'm contributing heavily to it. To give a since of scale, there's ~1000 custom fangames made in this: so pretty niche, but if your thing is Zelda it's got some real gems.
Most of my time has been spent practically rewriting the engine from just single-screen play areas (like Zelda 1) to be free-scrolling (like Zelda 3). I've also put lots of work into supporting all platforms (was just Windows; now it's also Mac/Linux/Web). And I've delved into tons of interesting programming projects while working on this: a deterministic record + replay testing system; a garbage collector for our custom scripting language; JIT compilers for x64 + WASM; a VS Code language server; the list goes on...
Anyhow, this month I'm trying to polish it up as much as I can so we can officially release the next major version.