Google will present a Windows "emulator" on Tuesday next week, during the Google for Games Developer Summit. One of the points on the program is namely "how to write a Windows emulator for Linux from scratch".
This may mean that Stadia will be able to run Windows games. If Google has actually created an emulator or a compatibility layer, it will be easier for developers to release their games for Stadia, as they do not have to port the games to Google's Linux-based machines.
For users, this could mean that the range is expanded at a faster pace than today and that more developers jump on the Stadia train.