“...the pilot episode of any TV series does more than just introduce great characters or tell a great story. It creates an engine powerful enough to launch every character in the series into a huge journey-- and a replicable structure for the series powerful enough to last many seasons.
But Stranger Things 1 has a particularly challenging structure to replicate in Stranger Things 2, because the whole structure of the first season is built around a simple problem that’s completely resolved by the time we get to the second season!
In Stranger Things 1, a little boy named Will is missing, and his merry band of friends friends need to come together in a real-world Dungeons & Dragons quest to find him.
Wrapped around this very simple structure are a bunch of wonderfully horrifying elements-- a creature that can only be seen in the shadows, a magical world called The Upside Down that’s only gradually revealing itself, a bunch of creepy-creepy operatives that are ready to kill to protect a secret that even they don’t understand, a mother communicating with her lost son through Christmas lights, and of course, Eleven, a little girl with magical powers who everyone seems to be hunting.
But by the time we get to Episode 1 of Season 2, Will is back (at least mostly) in the real world, so there’s no missing boy to build a structure around.
By the time we get to Season 2, the terrifying Demogorgon, which once was scary like the shark in Jaws, not for what we could see, but for what we couldn’t-- has not only been flushed from the shadows, but vanquished from them (at least mostly). So we need a new fin in the water to build the terror around.
By the time we get to Season 2, the world of The Upside Down, which we only barely understood, has now been entered and explored. So we need a new mystery in The Upside Down to build the world around.
By the time we get to Season 2, the mother, Joyce, can communicate with Will while driving carpool, so we need a new spiritual component that no one else understands to build the relationship around.
And by the time we get to Season 2, (at least as far as we know), Eleven is gone. So we need a new magical little girl to build the threat around.
In fact, by the time we get to Season 2, the only major structural element we still have going for us from the engine of Season 1 is the creepy operatives. But even they are a whole lot less interesting, now that we have some sense of who they are and what they do…
Which means the first episode of Season 2 has to do a lot more than replicate the engine of Season 1. It actually has to re-launch it…”