I totally get where you're coming from, Metal is much easier to program than raw Vulkan. I think a Metal-like API over "modern" APIs could be a good thing. There are things like the Vulkan Memory Allocator that help, but among other things that makes integration of different modules harder; this is especially nice on Metal because you can just allocate your own stuff off MTLDevice.

That said, there would be challenges, and big ones, the more you tried to make it actually Metal compatible. You'd need a C++ compiler for the shaders, and certain idioms (pointers, scalar types other than 32 bits) would be difficult to translate portably.

And as adamnemecek points out, the role of a friendly cross-platform GPU layer is largely being occupied by WebGPU (wgpu in particular in Rust-land). That'll be very comfortable for people used to Metal, as there's been lots of input from Apple folks. However, WGSL is much lower level than Metal Shading Language, and there are lots of important capabilities (subgroups, etc) missing that Metal and other advanced (>DX11) APIs can do.