Well we have Sekiro on Stadia which was developed by the same developer, so there's definitely a possibility that Elden Ring will be added in the future (no guarantee though). As cliche as this might be, the best bet for now is to submit feedback to Stadia (you can do it through the app) and also to Fromsoftware which is the developer of the game. The more people requesting it, the better chance it's happening.
------------------------------------------------------------------------------------------
Stadia Guides are not employed by Google. Opinions are my own.
