Support non-flatpak apps into "ecosystem"

[Beenman11]

Lets use Steam from its debian repository as an example.
The only Thing you get with non flatpak apps is an appmenu entry.
Steam doesnt show up in System settings -> Applications, and you can't see it in your dock as a background application if every window is currently closed.
I prefer using OS-native applications, especially for Steam, because from my experience Proton has a terrible performance if used through flatpak-Steam.
Therefore it would be really nice if there would be a way to add .deb / apt programs into the rest of ElementaryOS features :)