I’d like to call it:
Think in terms of the Kodi Remote, or KDE Connect.
It allows you to control your RetroArch/libretro features on your other device from your phone instead of switching to the menu every time.
It can range from using the keyboard, changing the volume, taking screenshots, starting recordings, switching shaders on the fly, configuring overlays more creatively, choosing your theme colours…
It should also come with a RetroPad option that allows you to use your phone as a controller for the other device itself (and possibly other supported systems).
And it’s so moddable, anyone can make a skin for it.
And if your friend uses the app too, you can use the app to more easily connect your devices with NetPlay (or something similar).
But that last part is a bit too ambitious. Basically, what do you think?