Microsoft has been planning for a Mouse and Keyboard support for Xbox One for some time now. The Minecraft Better Together game already supports mouse and keyboard. Xbox Platform VP Mike Ybarra confirmed that mouse and keyboard support for Xbox One will be coming soon.
He however had not revealed much details about the keyboard and mouse support for Xbox One. Microsoft has been already testing it with mixed input playlists on Gears of War 4 and the company wants the players to have a choice over who they compete against.
Mike also said that if any multiplayer game on Xbox is split between keyboard and mouse vs controller playlists this would cut player pools and reduce matchmaking speeds. Microsoft is currently letting developers choose if they want to implement these features.