An engine rewrite of a Japanese roguelike I played a lot[1]. I like Emacs, so I decided to see what would happen if I tried writing it in the style of what Steve Yegge calls "living systems", where all the code is interactively callable in-game and reusable in mods. There is no scripting layer, the implementation and extension language are one and the same (Lua). I like to think of the engine as a massive programming runtime with a bunch of libraries and functions made for the sole purpose of modding the game. You could whip up a scratch buffer and start tinkering around with the game state or prototyping new mods fairly quickly.

The engine is not general purpose either, it's specific to the quirks of the original game. The number of weird ideas that I could graft onto it keeps increasing with each week. Yet, without stability and feature parity with the original, it's a long way away from having those things.

Another downside is going back and playing the original now isn't as fun, because I keep thinking I'm playing the rewrite and expecting bugs to pop up at every corner. Working on a project like this for so long affects your perception of the end result in ways you can't easily unsee.

Also gets pretty lonely working on something alone for years you're not sure anyone will care about when it's playable.

[1] https://github.com/Ruin0x11/OpenNefia