This one is beyond obvious. RTS games were always meant to be played with mice and keyboards in mind; console controllers were ostensibly tacked on later to varying degrees of success (anybody remember Starcraft 64?).
Players have been begging for mouse control for Halo Wars 2 since it first landed, and this seems like a title that must be in the next crop of K+M-compatible announcements.
It's worth noting that players have actually been hacking together unsupported third-party solutions for this problem since the game released, but they usually get banned when playing online, so official mouse and keyboard support would be a godsend to the player base.