Rocket League will be one of the first titles to support the new cross-network play, and Microsoft notes there is an "open invitation for other networks to participate as well," strongly hinting that Sony or other console manufacturers could enable gamers to play their friends regardless of what device they're using.
If developers and console makers get behind Microsoft's support of cross-network play then it will change the shape of games in the future. Currently, console gamers are restricted to playing titles on the PlayStation Network or Xbox Live, and many pick an Xbox One or PS4 based on what their friends purchased. Opening up online multiplayer gaming across different platforms will remove a big restriction in console gaming, but it will also create a conversation around which titles should allow PC gamers to play against consoles. A mouse and keyboard can still have many advantages over a gamepad for first-person shooters, so it's a new challenge that game developers will have to balance for cross-network play.