I'd love to see a single UI that works across 4" phones and 7" tablets with gorilla glass, and 13" laptops and 10" convertibles with membrane keyboards, and 24" desktops with 101-keyboards, and 60" XBox Ones with controllers but I'm not holding my breath.
There are many benefits of a unified OS. If Xbox, Windows desktop, and Windows tablet all run under a single, unified OS, security updates can be pushed to all simultaneously, you can significantly reduce the amount of labor required to support all three, and developing cross-platform becomes much easier.
Buy why, oh why, do we need a unified OS? Desktop, Phone, Tablet, and TV all require different UI's. With Linux, we have essentially, a unified OS with different window/desktop managers and systems running on top. Why can't Windows be the same? You start your phone, Xbox, desktop, or whatever else, and the Windows kernel loads, and then it launches explorer.exe, xbox.exe, mobile.exe, or what have you, after that. It seems to me that it would be a lot easier to deal with a unified Windows like that. Such an approach would be future-proofing the OS, too, because when we inevitably get a new user environment in the future that Microsoft may want to expand to (e.g., automobiles), Microsoft can keep the core OS but just add a new window manager tailored for the specific environment. With our current approach, if Microsoft wanted to expand into automobiles, they'd have design a single, unified OS that works well enough for phone, tablet, car, tv, desktop, and laptop. Ridiculous!