Although the Windows Phone 8.1 update hasn't yet been released, Mary Jo Foley is saying that Microsoft is already working on the Threshold update. Details on what exactly the update will bring are scarce, but the idea is that Microsoft will be working to unify the Windows Store, Windows Phone Store, and the Xbox Store into a singular marketplace. There will also likely be better tools to attract developers to each platform.
Just like Microsoft's upcoming voice command/virtual assistant, Cortana, the name Threshold has been taken from the Halo universe. Threshold was the name of a planet in the earlier Halo games. No firm idea of when Microsoft is planning to release the next update to Windows, but Foley is saying it may not be until the spring of 2015.