I am against Dx12, Vulkan since it's unreasonably hard to achieve performance benefits and questionable if most monogame applications are even a good fit.
Plus it makes it near-impossible for 99% of the community to work with the source code, since it is much more complicated than dx11. Plus, we would need libraries like vulkansharp etc., which have not been tested extensively and are potentially unreliable.
Not mentioning the fact that most computers right now are not eligible in terms of hardware to even use dx12/vulkan and the fact that the target audience of 99% of monogame applications shouldn't need great hardware, since - let's be real - 2d is where monogame can outshine Unity and co.
Main benefits of the new APIs are potential async operations and better support for CPU multithreading requests, but it's hard to combine that with the default "true-and-tested" rendering paths of the classic XNA / Monogame application (aka: Update your simulation -> render stuff (with spritebatch) -> repeat).
I think if some masochist (me) wants to work with dx12 monogame might not be the right place.