It'd be really nice if the Series S|X would run a general emulator for Xbox and Xbox 360 games that don't get manual touch-ups/attention. That way they could get around the licensing issues of some games while having them be available. It'd be cool to see newer headlines saying something like, "Dead or Alive 4 receives texture updates" or "Project Gotham Racing is on sale on the digital store again and you won't believe the updates that it's receiving".
Cause if it does that, the first thing I'm testing are all of the DDR games. If they don't have timing issues, I'll be in love for like a minute. If they can support usb dance pads properly, I'll be in love forever.
Valve originally only allowed for whitelisted games to be played via Proton before letting users choose to use it on any Windows game. I'm really hoping that Microsoft allows for that floodgate to be opened. Just place some warnings and an indicator on what's been whitelisted and what isn't and they should be fine. You'll lose some visual updates and the ability to install the game to the drive (or at least to buy the game digitally in some cases if that's not an issue), but that would be fine.