My idea is to have a real looking or clay mockup like device frame when viewing prototypes like Invision studio is planning to do!
Are you looking to create this for shared prototypes or in-product preview?
Ermin Kadic commented
It would be a nice addition to select a device frame to view your mobile prototype on.
e.g. When in prototype mode, you can choose "device frame" (Pixel 3) and when you press "play", boom your design is in a device frame.