We don’t really know what happened to Hopper’s daughter
The first season of Stranger Things gave us a few hints. In one scene, we see Sara Hopper become suddenly frightened while playing with her parents outside. Then we see her presumably after chemotherapy while Hopper reads to her. We’re told she died of cancer but… did she really? David Harbour who plays Hopper (and will be the new Hellboy in an upcoming reboot!) told TV Guide:
"Hopper's daughter will be the primary focus of Season 2, but not in the way that you think it will be. Hopper's understanding of that relationship and Hopper's understanding of being a father and of being a man grows deeper and deeper in Season 2, and we get a lot of time to explore what that is, but it won't be in the literal way that you think it is. I don't want to tell you any more than that because it's good stuff."
Which sounds like Sara Hopper could be important in a plot way, or that Sara Hopper is just important in a thematic way. In Season 1, Hopper used the pain of the loss of his child to motivate his search for Will Byers, and his Eggo-stashing relationship with Eleven at the end of the first season had some people questioning if Eleven was secretly Hopper’s daughter somehow (unlikely). It seems more like the doctors observing Will Byers might be more sinister than they appear, perhaps like the doctors that treated Sara Hopper? If we went back to that first introduction in Season 1 and the reverse angle of the shocked little girl’s perspective, would there be a giant shadow monster in the clouds?
At the end of Season 1, Hopper’s loyalty remains in question: We saw him get in an unmarked car at the end, presumably having come to some deal with Hawkins National Lab to allow Joyce and himself access to the Upside Down portal. Investigating that portal and what came out of it could still be Hopper’s job, but it is just unclear where his loyalties lie beyond him feeling protective of children. Did Hawkins Lab know something about Sara that has kept him there? Was she One, Seven, or Ten?