The Xbox FSE has a compact "Game bar" in the overlay to launch pinned games / apps. It also has a means to view and select different active apps to focus on (basically the Alt + Tab menu).

My suggestion is to add something similar to the Steam / SteamOS overlay: a taskbar in the overlay that you can pin installed apps to, along with a button to manually add apps and a way to switch between apps neatly. Instead of a list of apps like current SteamOS or a horizontal line of apps like the Alt + Tab menu, active apps could fill up a radial menu of apps to easily navigate with a joystick / trackpad on the overlay.

The overall idea would be optional quick access to 3rd party apps like Discord or other launchers and make it a bit easier to mulitask in Steam game mode.