Google's Hangouts looks to have a new UI layout in the works, as a screenshot of the Hangouts for Chrome app with a new layout had recently shown up in the Google issue tracker showing off the new look. The screenshot has since been removed from the issue tracker, and there's no way of telling if Google will actually ever release the new design to the public but doing so would certainly make the app feel more streamlined. The interface is likely going through internal testing at the moment, and judging by the way things look it feels like it has a more material design style to it, most notably portrayed via the floating action button for starting a new hangout.
Since this is the Chrome version of the Hangouts application there is no guarantee that Google will bring the same style interface to the Android application if and when they apply this UI to the Chrome app, but it wouldn't make sense for them to skip out on an Android-based redesign since most of the things we see in material design are currently on Android. The new two-paned layout window also shows off some of the shadows that are present within material design apps and icons, as well as the ability to see both your conversations list and the individual conversation you're engaging in at the current moment side by side.
If implemented on the Android version, it could likely come to tablets only and potentially only when in landscape mode due to the size of the screen, although there is no way of knowing if this will ever make it out of a testing phase or not and which devices would support as there is no confirmation it will make it past the Chrome version if it's released. The new design carries the same green headers as before only though they're placed on the bottom in this image instead of on the top where they currently sit in both the Android and the Chrome version of the application. It also appears like they may have implemented a separate button in the header for video hangouts, which currently on the Chrome version of the app appears once you have selected someone to Hangout with when you're given the option to choose a Hangouts message or Hangouts video call. If you use Hangouts on Chrome, look for this to be a future layout design.