Halo Wars 2 is coming to the PC as well as the Xbox One, and it's a Play Anywhere game, meaning that if you buy it digitally on one platform, you'll have access to both. But it will not support cross-platform play: 343 Industries boss Dan Ayoub told Eurogamer that the studio had considered including the option, but it just didn't work out.
"Cross-play is something we were looking at early on in development," he said. "We've been in development since 2014. But as we looked at our timing and our design decisions, we focused on some different things that basically became Xbox Play Anywhere."
It's purely conjecture, but I wonder if difficulties maintaining balance between players using a controller, and those on mouse and keyboard, may also have been a factor in the decision to keep PC and Xbox players in their own separate sandboxes. During his appearance at the PC Gaming Show at E3, Ayoub didn't come out and say that a mouse/keyboard setup is inherently better for RTS games, but he did point out that Halo Wars 2 will take advantage of its differences in things like multiple groupings and the ability to quickly navigate the map and other aspects of the game.
Ayoub said cross-platform play could be incorporated “potentially down the line” in some future game in the series. “But in the Halo Wars 2 timeframe, we're focusing on Play Anywhere.”
Halo Wars 2 is set to come out on both platforms on February 21, 2017.