Windows 8.1 isn't even here yet and rumors regarding the next version(s) of Windows are beginning to surface on the web. Today, a report from Mary Jo Foley claims that Microsoft originally had plans to release the next major version of Windows in Fall 2014, but this doesn't seem to be the case anymore.
According to the report, Microsoft is working on releasing the next major version of Windows in 2015, Spring 2015 to be exact. We already know about an upcoming update for Windows 8.1 which is set to debut in Spring 2014, but this isn't an entire new OS.
The 2015 release of Windows will be the next 'major' version of Windows, (Windows 9 maybe?) The 2015 release will bring closer both Windows and Windows Phone OSes, and signifies that the recent Windows Store merge rumor is indeed legitimate.
Mary Jo Foley also claims that Windows RT and Windows Phone will be one OS by the time 2015 rolls around, which also lines up with recent rumors regarding the merge between those two operating systems as well.
It appears that Microsoft is aiming to make Spring the new time of year for Windows launches. Usually Microsoft would launch a new version of Windows in the Fall of the year, much like Windows 8.1 and it's predecessors.
Of course, as this information comes from unofficial sources, we ask you to read with a grain of salt.