DirectX is exclusive to the Windows platform. At this point, it's probably deeply tied into Windows. I don't see how you can make that bet.
Imagination Technologies is a near 40 year old British silicon IP company that has been doing GPUs for quite some time, just not ones supporting DX up until now, and it has nothing to do with MS (in terms of ownership / rights / etc).
[1] https://learn.microsoft.com/en-us/windows/win32/direct3d11/o...
Similarly, Microsoft would need to release the non GPU specific bits for macos to fit the same model.
In kernel space you as the hardware manufacturer implement a display miniport driver, and heavily lean on dxgkrnl.sys
https://learn.microsoft.com/en-us/windows-hardware/drivers/d...
And implement interfaces like this one for D3D12 https://learn.microsoft.com/en-us/windows-hardware/drivers/d...
There's quite a bit though and it's kind of spread across several sections on MSDN.
Of course the idea of Apple switching from Metal to D3D is rubbish, but a Proton-like solution for macOS totally makes sense (maybe the "Game Porting Toolkit" will be exactly that eventually).