WordPress 4.6 is now in its first release candidate stage. That means that the beta period is over, and though the team thinks they are, “done, with millions of users and thousands of plugins and themes, it’s possible we’ve missed something.”
In other words, nothing (aside from fixes based on feedback) will change between now and release, and it's important for plugin and theme authors to test RC 1 against your own code to try and find any outstanding issues, and to ensure compatibility.
The target date for release remains August 16th, which offers a few weeks for additional release candidates and testing.
Additionally, be sure to check out the WordPress 4.6 field guide, which has yet again been prepared by Aaron Jorbin. This is a highly useful developer's guide to WordPress 4.6, with a list of major and minor changes to WordPress this release, including technical backstory, links, and tickets you can follow up on. This is always the most useful post to me prior to an official major release.