I would like just to reiterate my concern on Direct3D11/Direct3D12 interop support. The main problem today with DirectX integration is not the lack of interop, but the fact that the composition of a Direct3D surface is inefficient (the double buffering and lock make the Direct3D stalls at the framerate of WPF, plus if the Direct3D part is running slower than the WPF framerate, the whole WPF experience is slowing down). It is thus today impossible to efficiently use Direct3D in a WPF application, and I have abandoned D3DImage in favor of HWND incrustation (so with the restriction of not being able to put overlay XAML content).
So using Direct3D11/Direct3D12 won't change anything if WPF keeps the same integration design.
I would prefer WPF embracing DirectComposition (though that would be maybe much more work) that would fix most of our concern with WPF/Direct3D integration.