“Stranger Things” ended its third season with a big cliffhanger and very little hint, leaving fans to conceptualize various theories about what’s in store for the next season.

Several guesses on the identity of “The American” – a mysterious prisoner being held by the Russians according to Season’s 3’s finale – have surfaced online. Some of these theories point to Jim Hopper (David Harbour), Dr. Brenner (Matthew Modine), Murray Bauman (Brett Gelman) and even the unexpected Ted Wheeler (Joe Chrest) as the kidnapped American.

Reddit user “xanshue” thinks that antagonist Billy Hargrove (Dacre Montgomery) is “The American” – a theory that is less popular among others because Hargrove had already been killed by the Mind Flayer.

“What if at the end of [Season 3] the "American" was Billy? When Billy went [to] the Upside Down he saw a clone of himself, perhaps they somehow switched places and Billy escaped by creating a new gate somehow,” said the user. “It would explain how that demadog was alive in the Russian lab as well.”

The speculating fan also mentioned irregularities on other theories.

“I don't see how it would be Dr. Brenner because Joyce would've mentioned him being kidnapped by Russians” user “xanshue” continued. “Also, the ending of S3 (Hopper's note), was really sad and I don't think Netflix would just bring him back.”

Fellow fans quickly added their own perspectives to point out loopholes to the theory. Many of the commenting users are in favor of Dr. Brenner as “The American”."

“My theory is Hopper is trapped in the upside down & the American is the original scientist from season 1, Dr Martin Brenner, played by Matthew Modine,” said Reddit user “lordmulgar”. I don't recall ever seeing his death or corpse in all the carnage.

“In [Season 2 Episode 7] when Eleven visits her sister, they go to one of brenner’s employees house… He said Dr. Brenner was still alive and he could take them to him,” user “4-children” added. “There is a large possibility that the American prisoner is Dr. Brenner because we didn’t see him die.”

As fans await the premiere of Season 4, the Dr. Brenner and Jim Hopper theories are becoming more popular but other unique theories have popped up.

Reddit user Sergeantbusiness wrote that Ted Wheeler might be “The American” and he was kidnapped for being a CIA agent – an unlikely guess because Wheeler’s character had minimal exposure in the past seasons. The theory was made based on Wheeler’s suspicious six-figure income and calm demeanor around CIA agents he interacted with on previous episodes.

“Stranger Things” Season 4’s premiere date is yet to be announced. Its first episode is entitled “Chapter One: The Hellfire Club”, an “X-Men” comic book reference.

Stranger Things "Stranger Things" may end with Season 4. Stranger Things/Facebook