Super interesting project. Been looking into template generation and uber shaders to make coding shaders easier for the game engine Ursina. I'm going to bring this to their attention.
Multiple targets is definitely the plan, and GLSL is my next priority. Metashade can currently generate HLSL for the DX12 version of https://github.com/ppenenko/glTFSample/tree/metashade_demo, and there's also a Vulkan version of that demo using GLSL. So implementing GLSL generation for that would be a great proof of concept and a test bed: HLSL and GLSL generated from single source and producing identical rendering results.
The diagram on slide 31 of my presentation shows how implementation is currently inherited between the Metashade packages. The future GLSL generators should inherit common functionality from the "rtsl" package, just like the existing HLSL generators. However, I expect heavy refactoring to be necessary because I was initially targeting just one language and so not all code is implemented at the appropriate level.
Would it be easy to add another target like glsl?