We can assume a few things here...
THE UPSIDE DOWN
This might seem the most obvious and maybe the most done... but the Upside Down has been a problem for these people since season one. Perhaps season four will explore the Upside Down further, and maybe, finally, get rid of it for good?
Let's not forget the Russian's input in this whole debacle. At the end of season three, we're transported to some Russian base where they're holding an "American". We're not giving up hope that the "American" in question is in fact Chief Hopper (David Harbour). Maybe the gang are going to Russia to save him?
WHEREVER THE HELL THE BYERS + ELEVEN HAVE GONE
What we also see at the end of season three is Joyce Byers (Winona Ryder) take her kids Will (Noah Schnapp), Jonathan (Charlie Heaton) and now adopted Eleven (Millie Bobby Brown) the hell out of Hawkins. Maybe the show will follow their journey away from the crazy town we've grown to... accept.
All we can hope is that it doesn't take the turn it did in season two when they ventured out of Hawkins for a single episode, following Eleven's self-discovery. That standalone episode sucked and we can all agree on that.
Along with announcing the new season, Netflix has added used this opportunity to announce a new deal with co-creators Matt and Ross Duffer. The brothers have signed a multiyear overall film and series deal with Netflix!
“The Duffer Brothers have captivated viewers around the world with Stranger Things and we’re thrilled to expand our relationship with them to bring their vivid imaginations to other film and series projects our members will love,” said Netflix's chief content officer Ted Sarandos in a press release. “We can’t wait to see what The Duffer Brothers have in store when they step outside the world of The Upside Down.”
BRING IT ON!