For Insiders on the 'fast' ring, there's the second 'Redstone 2' branch build now available, build 14915.1000. This branch isn't due for public release until Spring 2017, but you can get a head start by upgrading now and then following Microsoft's progress through August-March. There's little new on the surface yet, mind you, aside from the new system sound effects that came in for 14905, with the main changes being under the hood, with a new version of 'OneCore' the core code that underpins all modern versions of Windows.

From the Microsoft blog post announcing the build:

Now, onto the news you all have been waiting for! Today we are excited to be releasing Windows 10 Insider Preview Build 14915 for PC and Mobile to Windows Insiders in the Fast ring.

Improvements and fixes for Mobile

• We fixed the issue causing apps to get stuck in a pending state if you move apps between a SD card and internal storage (either direction).
• We fixed the issue causing Cortana’s text to speech capabilities to not work. Cortana should be able to read text messages out loud for you, tell jokes, sing, or give verbal prompts as expected.
• Changing functionality via Settings > Update & security > Windows Insider Program such as your ring settings should now work again on this build. For more information – see this forum post.
• We fixed a compatibility issue from a recent platform change that causes apps such as Yahoo Mail, Trivia Crack, Google and the Skype Translator Preview app to crash.
• We made a number of improvements to translations, including fixing an issue with Store app install notifications, where the wrong verb was used in German when letting you know to check out the app.
• We have updated “You need to fix your MSA” notification toast logic, so it will no longer notify you multiple times in quick succession.
• We fixed an issue where examining Microsoft Edge’s advanced settings would result in subsequent downloads in Edge saving to the default locations, rather the preferred locations set under Settings > System > Storage.
• We fixed an issue for Continuum-capable devices that could result in being temporarily unable to connect to a previously paired Windows 10 PC.
• We have updated the settings for sounds so that .mp3 or .wma files downloaded straight from OneDrive or copied from File Explorer to the phone’s Ringtones folder will now display automatically in the list of available sounds for ringtones, alarms and notifications.

All my Fast ring devices are downloading and updating as I write this. 

Of course, in the fulness of time we'll need a summary of what's new going from Redstone to Redstone 2, but we won't have a full feature set or changelog until at least Christmas 2016. Settle down for the long haul, everyone....!