Starts on PC, then it'll move on mobile devices. That's what we know so far.
Xbox has never been mentioned. I'd personally doubt they'd go on consoles this time around, but that's me.
We don't even know if this will be tied to steam, and the current information roaming around suggests a separate client.
We do know it's written in Unity. This engine is supported by quite a lot of platforms, including Windows, OSX, Linux (please please please!!), Browser plugins, Android, iOS, Windows Phone, Blackberry, XBox One, XBox 360, PS4, PS3, PSP Vita, Playstation Mobile, WiiU, Wii, some Smart TV's and also supports VR.