We have seen 4 developer preview builds of Android N. So far this doesn't seem to be a huge design overhaul in the UI. There actually aren't a ton of new features in Android N. Android Police is reporting that we will see a redesign of the Navigation Bar in the final build of Android N.
The image shows a render of what the new nav bar is expected to look like. The home button will animate when long pressed, and appears to look similar to the Nexus logo. This makes me wonder if the redesigned nav bar will be specifically for Nexus devices. Other than the home button the back and recent apps button are filled in which looks nice.
via Android Police