http://www.pcgamer.com/killer-instinct-on-steam-will-support-cross-play-with-windows-10-and-xbox-one/

Microsoft announced in June that the free-to-play, Windows Store-exclusive fighting game Killer Instinct would be coming to Steam later in the year. That hasn't happened yet, but community manager Rukari "rukizzel" Austin revealed today in the KI forums that when it does, the Steam edition will be cross-play compatible with the Windows Store and Xbox One versions. 

"When you download the latest Killer Instinct Content Update, if you haven’t done so already, you’ll undoubtedly notice something peculiar, so we wanted to give you a heads up at what’s to come before you spotted it in the wild. Tucked away in this CU is a switch that will enable certain feature you’ve all been clamoring for, and that feature is indeed cross-play!" he wrote.