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