Just some random thoughts after a week or so since I finished S3. During the next (and final?) season they really need to expand the scope globally because the story desperately needs that. They seem to have all the pieces in the right places: Dr. Owens with the government now know that the situation is most definitely not contained, the Russians are apparently still at it with an American prisoner as a cherry on top and my guess would be that Hopper is not that prisoner: he jumped through the gate and is now trapped in the Upside Down for who knows how long. I feel like we need to find out what exactly is the nature of the Upside Down. For instance the next season might be easily be set in 1987 and I think we all remember very well what happened back then and how it could be tied to the story.
Unfortunately I think that the biggest problem with the show are the kids who are fan favorites. They keep the story tied down to a smaller scale which was fine back in S1 but now it's a huge restriction. But then again, there might be easy workarounds for that... after all there is a reason why I'm not a writer myself.