Today, Microsoft pushed a new build to Office Insiders on the Slow ring using Windows desktop. As the company's @OfficeInsider account promised on Twitter, it's quite a significant update. It brings the version number to 1707 (which implies that it should be finalized for production this month), and the build number is 8326.xxxx.
In fact, it's quite a jump from the last one, which was only released four days ago.
One of the big new features is that you can use 3D models in Word, Excel, PowerPoint, and Outlook "to increase visual and creative impact in your documents, workbooks, presentations, and messages." There's also a morph transition in PowerPoint that will offer 'cinematographic animations' between slides.
Collaborative editing is a feature that's been added to Excel. This allows multiple users to edit a spreadsheet at the same time. You can find out more about that here.
Another thing that Microsoft highlighted in the release notes is AutoSave for Word, Excel, and PowerPoint, saying, "Changes are automatically saved for documents, workbooks, and presentations stored in the cloud, and you can see others' updates in seconds."
That's really just the tip of the iceberg. Outlook users will see a new Conversational view, improvements to cloud attachments, and you'll see suggestions with pictures in the 'To:' line when composing an email. Word users can now edit using the Learning Tools feature, and can create and edit math equations.
If you want to check out the full release notes, you can find them here. To sign up for Office Insider builds, you'll need to be a paid Office 365 subscriber, and then you can open any Office app and go to File -> Account to turn on Insider Previews.