After several apps updates with Material design ir seems like Google will include Google Hangout on this list of updates.
Some screenshots have leaked showing an upcoming version of the Hangouts app with Material design. These updates include a new, less vibrant green color that extends to the status bar, an updated navigation drawer, a hamburger button that’s slid out from the side of the screen and some updated iconography. The app’s icon suffered some Materialized as well.
The Hangouts new look is really more Material than the existing version. There’s no information on when this update will be released but we will let you know!