Unity's version numbering is changing from a major update model to a yearly system starting next year. Unity 5.6 will be the last release to use the old system, so there'll never be a Unity 6.
Instead of "Unity 6", the next iteration of Unity will be called "Unity 2017." Future updates to Unity 2017 will be labeled "2017.x." This new version nomenclature follows the narrative Unity CTO Joachim Ante gave earlier this year that there would be fewer major overhauls of the Unity engine in the future.
The Unity 2017 beta is scheduled to release in April, with more details to be discussed at GDC 2017. The Unity public roadmap already includes adding support for Nintendo Switch, new storytelling tools called Timeline, and optimizations to the core engine.
Unity 5.6 is itself a beta, with the full release slated for March, more than likely coinciding with GDC. Unity 5.6 will add support for Google Daydream, Facebook Gameroom, new progressive lightmapping, an in-editor video player, and Vulkan API support.
Unity continues to be a huge part of the game development world. It's possible the increased flow of updates and support to the Unity engine will continue to make it an even more attractive tool of choice for game development. Unity's change to a subscription model this year made them a bit less appealing compared to Unreal and CryEngine's free and play-what-you-want models, so perhaps more regular updates and feature additions will make Unity users feel like they're getting more bang for their buck.
Jason Faulkner posted a new article, There Will Be No Unity 6 Because of Switch to Date-Based Update Model
Switch upper case huh