Yes this is what I've been thinking! There are clearly five people in hazmat suits walking in when everyone else is running out and they seem fairly calm (dare I say calculated) - they certainly look to have youthful faces. The back to the Future hints were there and they always put Easter eggs for upcoming seasons in the episodes.
It's an easier solution to how hop survives (if the kids go back and save him via time travel). The alternative theories are plausible too but I get stuck on how hop could live in the upside down for an extended period of time, or make it all the way Russia where another gate might be (from what we've seen the upside down is a direct replica of our world so how the heck would he wander around and find another exit)... The time travel idea is cleaner than somehow transporting hop to Russia and having the kids execute an international rescue mission.
My guess is the guy in Russia is definitely Brenner, lesser odds that it's Murray. And hop survives some other way that we haven't seen yet. I really don't see him just wandering around the upside down avoiding the demo dogs and other monsters that live there, with no food or water for as long as it takes el to get her powers back. But he's definitely not dead. That's very clear with all the evidence that's already been provided (no body, a conveniently placed ladder, Peter Gabriel's heroes song, 6 seconds of black where anything could happen, and the hazmat team seems like a really huge clue here with the timing. The more I think about it, it feels like a very "on purpose" scene, it's totally unnecessary to have that scene at that time for the sake of the end of the season, it establishes nothing and only gives us another view of the key. So I think it's totally setting something up for next season.
They introduced Erica as a math genius and Robin seemed to have a good grasp on back to the Future, and suzzie seems pretty smart. I'd love to see them venture into time travel... it's also a category/type (and a pretty big one I might add) of 80s movies/tv that they haven't yet worked into the series. I'm here for it.