Billy Hargrove met his demise in Stranger Things season 3, but that doesn't mean his time on the series needs to be completely over. The character, played by Dacre Montgomery, made his series debut in season 2 and has been an antagonist of varying degrees ever since. Here's how Billy could return in season 4 without ruining his redemption.
Billy and his stepsister, Max, were introduced at the beginning of season 2 when their family moved to Hawkins from California. As the new stereotypical "cool guy," Billy instantly entered a rivalry with Steve. He also revealed himself as a bully, especially in his treatment of Max. She eventually got the upper hand, but matters became serious in season 3 when Billy was attacked by the Mind Flayer and chosen as the new host. He then built an army for the Mind Flayer and set his sights on Eleven. With her help, Billy was able to regain his control from the Mind Flayer and made the heroic sacrifice to save Eleven, his sister, and their friends before succumbing to his injuries.
In all likelihood, Billy will stay dead with no hope of a future in Stranger Things, but the same fate was meant for Jim Hopper who was recently revealed to be alive in the season 4 teaser trailer. Many fans are now wondering if Billy could be the next "dead" character to have miraculously lived through the season 3 finale.
The Mind Flayer had complete control over Billy, but viewers aren't fully aware of the Upside Down and its powers. It's also unclear what happened to Billy's body after the events in Starcourt Mall. Maybe he can be revived or the Mind Flayer has the ability to clone hosts that it has imprinted on in the Upside Down. The show can also elect to pick more logical avenues that still give Billy a role in season 4.
There's no news regarding Montgomery returning in any capacity for the forthcoming installment of the hit Netflix series, but that doesn't mean the actor can't appear in a limited capacity. Since Max will presumably deal with losing her stepbrother and witnessing his death, flashbacks provide an opportunity for the series to dive back into their family's complicated relationship. This method was used in season 3 with Bob Newby even though the character was killed off the previous season.
If the series focuses on flashbacks or some type of an alternate reality, it won't diminish Billy's redemptive action in facing the Mind Flayer before being stabbed to death. In the small chance that the real Billy comes back to the current timeline, he will undoubtedly continue that redemption arc and finally join the group as a member of the good guys, something that many viewers had hoped to see. The options are truly endless, and with a show like Stranger Things, anything is possible.