You mean one of the hubs with a screen? I guess that's...a thing that could exist, although it seems a little "just because" and not cause it would be a worthwhile use case.
Since you can Cast a game from the app to a TV/Chromecast, and the Hubs use the same Casting protocol as all other Google products, I would think with fair confidence that it is at least technically possible for Stadia to be casted to a Nest Hub -- but I would also agree with others here that something like that would be on the very low priority list. Right now Stadia only supports casting to Chromecast Ultras, so casting to Nest Hubs would only be after they lift that restriction. However, casting to Android TVs, regular Chromecasts, and other devices like that are more important.