In early December of last year, the first mention was made of Windows Threshold, the purported successor to Windows 8.1. Paul Thurrott, a blogger with lots of news and rumours on Windows, claims that Threshold might actually be renamed to Windws 9 in an attempt to leave the negative image behind that Windows 8 has with many consumers. Threshold (or Windows 9) is rumoured to bring back the start button, will allow metro applications to run in the desktop environment, and will bring different platforms running Microsoft operating systems closer together.
In April, the Build 2014 event is held, where Microsoft will undoubtedly reveal more information. It's expected however that there will not be a preview version shown of Threshold/Windows 9 at the event, which doesn't come as a surprise since development is reported to start in late April of 2014.
It's rumoured that Threshold will produce three milestones, with each milestone being a new version of an operating system. What the individual milestones will be cjalled and whether they'll actually be available to consumers is still unknown. Lastly, it is claimed that Microsoft will release Windows 9 / Threshold in April of 2015.
Windows "Threshold" to become Windows 9 in April 2015? image source: digitaltrends.com