I've been working on a 3D voxel-based game engine for like 10 years in my spare time. The most recent big job has been to port the world gen and editor to the GPU, which has had some pretty cute knock-on effects. The most interesting is you can hot-reload the world gen shaders and out pop your changes on the screen, like a voxel version of shadertoy.

https://github.com/scallyw4g/bonsai

I also wrote a metaprogramming language which generates a lot of the editor UI for the engine. It's a bespoke C parser that supports a small subset of C++, which is exposed to the user through a 'scripting-like' language you embed directly in your source files. I wrote it as a replacement for C++ templates and in my completely unbiased opinion it is WAY better.

https://github.com/scallyw4g/poof