I am hoping that it's Brenner in the cell and that Hopper is stuck in the Upside down.
To me that makes much more sense for Hopper, I mean how the fuck else would he have survived that? The only logical thing was for him to jump into the rift before it closes, there was absolutely no time for him to get out or somehow for soldiers to grab him and take him away before then, literally just seconds sto spare so him jumping into the rift makes the most sense.
Also this allows Brenner to come back, we know he's alive and that makes the most sense for his character, to have defected or been captured and the knowledge he has would make him very valuable as a prisoner to them and how they've learned about all of this stuff.
Are the Byers and Eleven out of Hawkins and thus of the future story? The Byers, Joyce, Will and Jonathan, I could see but Eleven still seems relevant even without her powers. Just by long distance radio then?
No way they are getting rid of Joyce/Will/Jonathan and especially Eleven.
Remember Eleven talked about visiting for thanksgiving/Christmas and things.
I'd say the next season might very well be Christmas themed (a la Gremlins). That'd definitely be a good angle to play off of.
The setup being that Brenner comes back, then they find out that Hopper is alive and in the upside down, so Eleven has to go back to Brenner to try and figure out why her powers aren't working so she can open the rift to get Hopper back.