Microsoft's follow-up to Windows 8.1 will is said to be codenamed "Threshold." This according to ZDNet, which cites sources within the company as saying that the next major Windows update will come in the year 2015. The "Threshold" update is said to cover not only Windows and Windows Phone devices, but also Microsoft's recently released Xox One console.
The Xbox One OS, Windows 8.1 OS, and Windows Phone 8 OS share a common Windows NT core, so Threshold will be a continuation of Microsoft's scheme to bring all of its platforms into synchronous operation. ZDNet notes that Microsoft is working to have one app store across all of its platforms, something it has halfway accomplished with Windows RT and Windows 8, but a feat that will require both Windows Phone and the Xbox One OS in order to be complete.
Threshold will push a number of "high value activities" across Microsoft's platforms. These include Office functions, as well as perhaps a "Remix" digital storytelling app. This would give users access to critical Microsoft-built features across any number of devices, including their phones, tablets, notebooks, and desktops.
Threshold is said to be slated for a Spring 2015 release. Before that, though, Microsoft will likely deliver on its first big update to Windows 8.1, known as Windows 8.1 Update 1. That update is expected to arrive in 2014 in the spring quarter. Around that time, Microsoft should also be rolling out a big update for Windows Phone 8, dubbed Windows Phone 8.1.