There were several rumors about Windows 9 coming soon, while some believed a second update for Windows 8 titled 8.2 was around. According to Paul Thurrott , the project "Threshold" is actually a codename for the next Windows, most likely to be called Windows 9. Microsoft will announce it's vision for the future of Windows at the BUILD developer conference in April, 2014.
Paul Thurrott has wrote on his blog that:
But Threshold is more important than any specific updates. Windows 8 is tanking harder than Microsoft is comfortable discussing in public, and the latest release, Windows 8.1, which is a substantial and free upgrade with major improvements over the original release, is in use on less than 25 million PCs at the moment. That's a disaster, and Threshold needs to strike a better balance between meeting the needs of over a billion traditional PC users while enticing users to adopt this new Windows on new types of personal computing devices. In short, it needs to be everything that Windows 8 is not.
According to Paul Thurrott, Microsoft will announce Windows 9 at the BUILD conference in April, 2014 and ship Windows 9 in April, 2015.