Fans have been guessing the identity of the American teased at the end-credits scene of Season 3 and most theories point to Chief Hopper. A new theory pulls Billy Hargrove back from the dead in "Stranger Things" Season 4 as the American.
There is no way that Billy (Dacre Montgomery is still alive after the Mind Flayer killed him. He sacrificed his life to save Eleven and viewers saw him die a brutal and gruesome death (see video below).
However, Screenrant pointed out a few good reasons why resurrecting Billy in "Stranger Things" Season 4 as the American would be a good story twist. It would definitely throw viewers a "dramatic curveball." Creators Matt and Ross Duffer have been known to add a few surprising twists and turns in their show so why not have the dead Billy return as the American for an added "shock value."
Bringing Billy back would extend his redemption story from Season 3. In the finale, he broke free from the Mind Flayer's control after Eleven (Millie Bobby Brown) saw through his past and understood why he became a bully.
Billy's father physically abused him when he was a child and he carried the anger he felt toward him into adulthood. He showed his pain by hurting other people. This was shown in the Season 3 episode called "The Gate."
If Billy was to return in "Stranger Things" Season 4 as the American, then it would give him the chance to get back at his father, Neil. It would open a backstory to his relationship with his dad and give him the justice he deserves. Likewise, it would give closure to Billy's story arc and redemption.
Moreover, there is the possibility that Billy is in the Upside Down and Hopper jumped in to save him during the explosion at Starcourt Mall.
It is anyone's guess really if Billy will return in "Stranger Things" Season 4. It is interesting to point out the similarity between Will Byers and Billy. Both had awful father figures although they grew up differently. Will and Billy were also mind-controlled by the monsters from the Upside Down in Season 2 and Season 3, respectively. Will broke free from the monster's control and survived, so this gives the fans hope that Billy is alive somewhere too.