DirectX Tool Kit currently supports DirectX 11.x. While not everything in the tool kit is applicable to DirectX 12, a subset of the graphics functionality makes sense.
DirectX 12 has a very different approach to state and shaders, so perhaps that would be best done by implementing a PSOFactory. This would likely be used in place of CommonStates and Effects for DirectX 12.
DirectXHelpers12 would be a good place for the new SetDebugObjectName template and an UpdateSubResource equivalent.
Also a GenerateMips helper since 'auto-gen' mips is not a DX12 feature.
A LinearAllocator for handling communication with the GPU is also essential.