will it be possible to play games from other platforms such as battel.net or steam?
Only if they get ported to stadia, this is is trying to be it's own console type thing in the cloud, sounds like GeForce now is what your after.
@Subraaa :
As @ARGAH has indicated already, there are no immediate plans that have been announced to port either Steam or Battle.net into Stadia unless individual titles get a refresh and are released in such a way to support Stadia.
Ubisoft did make an announcement that they intend to bring Uplay Plus over to Stadia at some point in 2020, but no details have been released yet on which titles they intend to make available, what the additional cost will be for the subscription, or indeed when in 2020 exactly that will be rolled out.