The 1st CTP (Community Technical Preview) for BizTalk 2016 has been released today.
Whilst you have to sign up to Microsoft Connect to access the CTP, Microsoft have
published the release notes for the CTP here: BizTalk
2016 Release Notes
These release notes will be added to as the product gets closer to release.
For more details, including instructions on how to sign up, go here: https://blogs.msdn.microsoft.com/biztalk_server_team_blog/2016/03/30/announcing-biztalk-server-2016-community-technology-preview-1/