Looks like Office 2016 Developer and IT Pro review was announced yonder at the Office blog today.
Check it out – http://blogs.office.com/2015/03/16/announcing-the-office-2016-it-pro-and-developer-preview/
To participate you’re routed over to Microsoft Connect here – http://go.microsoft.com/fwlink/?LinkID=528801
If you’re excited about what’s next, definitely give it a whirl – be mindful though that per the article “the early build doesn’t contain all the features we’re planning to ship in the final product.” So what does that mean? More than likely Microsoft will be revving this over the next few weeks and months.