There is no magical "next gen" sauce that would make sense for a AAA game like Assassin's Creed Valhalla, Cyberpunk, Control, or Doom Eternal to run at native 4k on Series S in BC mode, while The Medium runs at 1080p natively. Minimal RT implementation isn't going to give you that kind of performance hit.
However, like I said above a few of these cross-gen games will be interesting case studies. Next-gen enhanced Control running at 1080p/30 with RT effects vs One X "last gen" running at 4k30. Or AC:V running at 1080p60 (presumably) on Series S, vs the One X version at 4k30, with varying levels of assets and LoD.