first of all as a non-debian user does debian have decent emulator support or decent ui emulators that are fine. Second of all would it be possible to play and enjoy playing on the librem 5 emulated games? it would be awesome if possible! and is it possible to have 3rd party support for external devices on librem? i.e. controllers or things of that sort like android?
Gnome Boxes are great and the one I use, so long as you can plug it into whatever port ends up on the libre it should be fine. What games are you thinking of emulating?
There is also GNOME Games, which I believe integrates with libretro cores and provides emulation for lots of older consoles and handhelds.
snes gba gbc maybe some ds