Microsoft is believed to be on track to release Windows 9, the next major version of the desktop operating system, in April 2015 and is expected to make an announcement at BUILD 2014.
Paul Thurrott, known for revealing information about Microsoft products, has published on his website, SuperSite for Windows, a detailed roadmap leading to the Windows 9 release in April 2015. In order to distance itself from the bad reputation of Windows 8 with the masses, Microsoft is reportedly planning to move on to the Windows 9 moniker with the next major release.
According to earlier reports, "Threshold" was believed to be an update to Windows 8 following "Blue", however, the new report suggests that it will be a different release altogether. According to Thurrott's report, Microsoft is readying "Metro 2.0," to advance the Modern UI further to be more mature and usable with the next release.
Paul expects Microsoft to make an announcement regarding the development of Windows 9 at BUILD 2014 in April of this year, but won't make the pre-alpha versions available to developers at that time. Windows 9 is expected to have 3 milestone releases before being released to manufacturing.
Source: SuperSite for Windows