It's just me or RetroArch developed the best FLOSS alternative to proprietary UI? You have a nice overlay, CRT shaders, bezels, some netplay function, customizable interface. All of this completely free and super fast.
Wouldn't be nice to also launch non-emulated games from here?
A lot of what you list is built into the emulation cores and are not just UI features.
For games that don't use their emulation cores:
They would need to come up with a steam-like injected overlay approach
CRT shaders are probably a no go without engine injection (similar to reshade)
Bezels could maybe work with stacked borderless windows, but again it would be an entirely new approach compared to what they have now
Netplay would also need something new and Hamachi-like and would only work with games that already have LAN support, patching support for online service based multiplayer would likely need per-game implementations
The customisable interface would probably not need any changes at least
Basically they would be building all the tough bits from scratch to do everything you're suggesting