The main impediment between cross-play between the Xbox One and the PC is the improved control capabilities on the PC by virtue of mouse and keyboard control available on that platform.
With the Xbox One basically a PC in a pretty box however and cross-play becoming a more common feature there is a clear demand for mouse and keyboard support on the system.
Head of Xbox, Phill Spencer, was asked whether the arrival of Halo Wars 2, coming on the 21st February, will bring support for keyboard and mouse on the Xbox One.
@ryan89778 It won't be there at launch but this is support we'd like to add as it comes to the Xbox platform.
— Phil Spencer (@XboxP3) January 30, 2017
Spencer clearly notes that Halo Wars 2 will not support the keyboard and mouse, but that they would like to add the feature “as it comes to the platform” which clearly suggests the feature is on the way to the Xbox One.
Halo Wars 2 will be a Play Anywhere title (ie you can purchase it for the Xbox and play it on your PC) but will not be a cross-play title at launch, but Microsoft has said they are looking to add the feature in future versions of the game.
Having keyboard and mouse support would certainly help level the playing field and thereby expand the reach of both the XBox One and PC.