While Microsoft has released eight Insider builds for the Redstone update, there hasn't been a whole lot that's new. Only a couple of those builds had any new features at all and those were minor, many of them finding their way into Production updates along the way anyway.
One of the most anticipated new features to come in the Redstone update is extensions for the Microsoft Edge browser. Microsoft hasn't been shy in talking about Edge extensions and there's even been code found in current builds to show that support is coming, but we've never known a specific timeframe, until now.
WinBeta has reported that Edge extensions are already ready to go. They're currently in testing and should be ready to go by the end of the month, "just in time for Build". The Insider build will be 14284, assuming that there's no show-stoppers with that build.
The report suggests that since the build is in internal testing, we could be just days away from a release to Insiders. This doesn't seem likely. Extensions are something that developers need to know, so it's likely something that Microsoft will talk about at their developer conference, Build, before releasing it.
In fact, Build is where Microsoft will likely talk about all of the major features coming in the Redstone update. OneDrive placeholders have been rumored, as have windowed apps in Continuum for phones, UWP on Xbox One, Cortana on Xbox One, and more.
If Microsoft's plans are anything like last year, there will be a build that's released a week or two after Build with a preview of all of the new features that they announce. Of course, in the world of prerelease software, anything can change at any time.