In their letter to fans, the Duffers said that there are still many more exciting stories to tell within the world of Stranger Things.
So far, nothing has been written, but there is a war going on between the Upside Down and the real world, with Vecna and Eleven as the generals.
The real villain of the Upside Down, Vecna, is defeated in the second part of the show, but he is not killed, and his disappearance almost certainly sets up his return in the final season. The way the Upside Down breaks through at the end of Stranger Things season 4 part 2 makes them less of a concern, as well as giving them a new story set-up. The story is far from over and while it may take some time to get over the events of Season 4, the hype to return to Hawkins and the Upside Down is real. After being defeated in battle, Vecna could disappear for even a period of years, skipping the main cast to high school graduation age, so that she could use the rifts from the Upside Down to attack the real world. It's unlikely that the final season of the show will be smaller than the first, but logic dictates the opposite.
It took almost two years to film Stranger Things season 4 even before the post-production stage started, and though lockdown will have played a part in that extended filming period, Stranger Things season 5 could still have a lengthy filming schedule.
In an open letter to fans, The Duffer Brothers confirmed that there will be a fifth season of Stranger Things. The four gates that opened throughout Hawkins connect at its center, opening rifts from which smoke and storms pour into the real world, prompting a mass exit from the town. The need to split it into two parts was due to the larger post-production requirements of season 5. We want you to stay with us as we finish this tale of a powerful girl named Eleven and her brave friends, of a broken police chief and a ferocious mom, of a small town called Hawkins and an alternate dimension called the Upside Down.
The reason for the time-jump is to help justify the fact that the teenage central cast are too old to play their ages.
The Duffers talked about other mysteries they could tell after they wrap up Eleven's story, and Ross Duffer has previously talked about when development may start. After the events of season 4 part 2 and the confrontation with Vecna, there was another rift caused by the villain. In season 4 part 2, there was enough time to tell Vecna's story, build the villain's mythology, and develop the dynamic between human cast members. Given the length of the season, the Duffers added the split in order to get it to you as soon as possible.
It was confirmed by Matt Duffer that the spin-off will be different from what anyone is expecting. It was not possible for the Duffers to shoot it back-to-back with season 4. Matthew Modine's Dr. Brenner may have survived "death" once, but he seems to be dead now, while Joseph Quinn's former living legend Eddie Munson died in the Upside Down, and Mason Dye is unlikely to return. Six years after the debut of the future Hellfire Club, the gang have gone through incredible hardship, losing friends and family members and saving the world on multiple occasions.
There's more to come for the Duffers, with a Stranger Things spin-off also promised.
The same problems that Game of thrones faced in its final season with pacing and trying to resolve storylines too quickly were avoided by the decision to extend to a fifth season. It makes sense that the next chapter of the show could be broken in two.
It wasn't done for cynical reasons, despite the fact that it was unpopular when the first part ended.