We are living in a fascinating era of fandom where fans have so much access to news and are able to communicate with each other on so many different areas online that the world of "fan theories" is exploding. Fan theories have always existed, of course, but the internet and social media (especially sites like Reddit where a large group of people can tackle a problem at once) have made it so much easier to come up with in-depth fan theories and send them all around the internet.
The downside of this, though, is that fans now sometimes get upset when their fan theories don't match with what the final film is, and sometimes the critique is not so much "what you did was bad" but "what you did did not match what we thought you would do, so we don't like it." That can be frustrating for filmmakers. However, when a project falters a bit, like the recent Justice League film, it is worth it to re-examine those older fan theories and see how many of them would probably have led to an improved Justice League film had the fan theories turned out to be correct.
15 STEPPENWOLF CONTROLLING SUPERMAN
One common fan theory was trying to tie in the existence of Steppenwolf in this movie to the resurrection of Superman by suggesting that Steppenwolf actually came to Earth specifically to bring Superman back from the dead (and then brainwash him to use him as a tool in Darkseid's army).
This would be an improvement over the actual film!
It would allow them to skip a lot of time involving Steppenwolf putting together Mother Boxes and cut right to the resurrection of Superman and presumably Superman being a villain for a period of time. It would give Steppenwolf a strong reason to be showing up at this precise moment in time and make it so that he is less the "big bad" in the movie and more of a secondary instigator (with an evil Superman being the main villain).
14 FLASHPOINT TAKING PLACE IN THE FILM
A clever theory that got a lot of attention was the idea that the "Flashpoint" concept would be introduced in this film. For instance, we saw the Flash come to Batman in a sort of dream in Batman v Superman: Dawn of Justice, and of course, lots of fan theories revolved around that dream! Well, what if we saw that happen in this film?
For instance, Superman returns from the dead and is evil. Things go horribly wrong and the world descends into chaos. The Flash then travels back in time and creates a new timeline. We see everything start over and the heroes manage to prevent the bad guys from taking control of Superman, using the information that they learned from Flash going back in time.
13 CRISIS ON INFINITE DC MOVIE AND TV EARTHS
A rather "out there" idea that would have still been absolutely amazing to see as a film experience was the theory that perhaps Justice League would tie into the Multiverse concept laid out in the DC "Arrowverse," and that the characters from the CW slate of DC TV shows could cross over with the film versions of their characters in Justice League.
Imagine the two Flashes racing on the opposite side of a wall, Earth-2 style!
What would really make this idea stand out is that it is something that only DC could pull off, as Marvel doesn't have the same slate of colorful superhero shows (other than AoS and its more grounded Netflix series). So this would have been a place for the DCEU to stand out from the rest of the pack. Alas, it was not meant to be.
12 PARALLAX BEING A VILLAIN IN THE FILM
The absence of Green Lantern in the original roster of the Justice League in the film (after DC had initially planned on using Green Lantern to literally launch this line of films) led to a number of fans trying to come up with reasons as to why there was no Green Lantern present in the film and one fan theory would have explained it nicely while setting up a sequel film.
The theory is that Parallax (last seen in the Green Lantern film) would have possessed Hal Jordan and perhaps some more Green Lanterns and thus made Earth off limits to new Green Lanterns. At the end of the film, the League could have learned of the existence of Parallax and set up a future film where the League fights against Parallax and the possessed Green Lanterns, with one presumably remaining on the team afterwards.
11 DARKSEID BEING IN THE FILM
Really, of all of the fan theories about Justice League, the one that seemed so obvious that it was hard to even consider it a theory -- since it seemed more like simply logic -- was that itwould feature Darkseid as the main antagonist. After all, Steppenwolf is barely even a villain in the DC Universe. He exists solely in the context of Darkseid. It would be like having Desaad in your film and not having Darkseid.
The one only exists in relation to the other.
However, as we all saw, the film did not end up having Darkseid in it, and it really should have. Even a cameo would have placed the narrative of the film into a more logical context. "Oh, okay, so this guy is working for that guy." Just saying the phrase "for Darkseid" once does not cut it.
10 LEX LUTHOR CREATES THE JUSTICE LEAGUE
This interesting fan theory played on the fact that the final edit of Batman v Superman: Dawn of Justice ended up removing a few notable scenes featuring Lex Luthor that would have made it clearer that he was far more in control of what was going on in the film than was first thought. In other words, his machinations were behind a great deal of the events of the movie. Meanwhile, we saw him talking to Steppenwolf at the end of the film.
Therefore, the theory would be that Luthor intentionally arranged the events of Justice League. Either on behalf of Darkseid, to get all of Earth's heroes together or, perhaps, as a secret double-cross to help protect his planet. He's evil, but he probably doesn't want Earth conquered by Apokolips, either! That would have been an interesting plot twist.
9 SUPERGIRL DEBUTING IN THE FILM
Fan theories have managed to expand in recent years thanks to fans going through trailers with a fine tooth comb. In the past, it would be more, "Hey, what's that amorphous blob in the background? Could it be ___?" Now, trailers are distinct enough that very small details are visible!
In the case of the Justice League trailer, it was more misleading.
There is a scene with someone wearing a red cape and a red-covered leg. Early on, the thought was that Superman was going to be wearing a black costume when he returned, so that led to the theory that perhaps Supergirl would show up in the film. Imagine if Superman went bad and Supergirl had to fight alongside the heroes against him? Now that would have been interesting.
8 SANDMAN BEING IN THE DREAMS OF VARIOUS HEROES
So far in the DCEU, one thing that seems to pop up a lot is characters having dreams. Obviously, the biggest example was the ominous dream that Batman had that the Flash showed up in to warn him about the future, but there were other dreams in the first two films in the DCEU. Heck, Hal and his brother even talk about dreaming about their dad in Green Lantern.
Therefore, one intriguing theory suggested that the Justice League movie could be used to introduce Dream of the Endless into the Extended Universe by revealing that it is he that is having these heroes have these very specific dreams. It could then set up Dream for his own film (presumably not otherwise directly connected to the DC Universe). Think of how cool Dream's appearance in Dark Nights Metal was! This could be just like that!
7 AN INJUSTICE PROLOGUE
Right or wrong, it is clear that the events of Batman v Superman: Dawn of Justice and especially Man of Steel caused some divisive reactions from fans who believed that the films were too dark. That led to a number of theories that perhaps the DC Extended Universe could sort of lean into those critiques a bit.
The way to do this would be revealing that all of these stories were really just set up for the Injustice: Gods Among Us video game series.
In that storyline, Superman is tricked into killing Lois Lane by the Joker. He murders the Joker and sets himself up as a tyrannical ruler of Earth. Batman rebels against him and the superheroes of Earth have to choose sides. The films so far really do fit in well into the Injustice universe.
6 TRAVELING TO APOKOLIPS TO RESCUE SUPERMAN
As noted before, the famous dream sequence from Batman v Superman: Dawn of Justice was the source of so many different fan theories. This is because there is so much open to interpretation in the dream. One of the most notable examples was the Omega symbol appearing on the land in the dream. That's obviously a Darkseid symbol, but it is also open to debate as to whether that scene is even set on Earth.
That led to a fan theory that perhaps Superman would be taken to Apokolips to be resuscitated and thus Batman and the newly formed Justice League would have to travel to Apokolips to save Superman and bring him back home. That would be quite the awesome adventure right there, although it would probably be a bit much to get to in one movie.
5 JUSTICE LEAGUE VS. SUICIDE SQUAD
In Suicide Squad, Amanda Waller sells herself and the Squad on the notion of what would happen if the next Superman was not a hero. Well, if Justice League originally had Superman turning evil, then that would have made Waller's comments pretty darn prescient, now wouldn't they? That would then open up an interesting avenue, as one fan theory suggested.
In it, the film would have served as a set-up to a Suicide Squad vs. Justice League movie.
The theory would be that while the Justice League movie would likely end with Superman back to being a good guy, the government might not be willing to take the chance that he doesn't turn bad again, so they might be willing to unleash the Suicide Squad on them. This could be set up by a post-credits cameo by Amanda Waller.
4 HAL JORDAN DEBUTING AT THE END
While this one could work with the previous fan theory about Parallax, a particular fan theory suggests that before he came to Earth, Steppenwolf specifically took care of the Green Lantern of Earth's Sector 2814. In this theory, he did by having Parallax possess him, thus tying in with the Parallax theory. Otherwise, it would just be a simple case of killing the Green Lanterns and preventing their rings from finding new hosts.
So the theory would be that some time in the film, we would meet Hal Jordan (presumably recast with a new actor) and then, at the end of the film, when whatever was preventing the Green Lantern ring from finding the host was no longer working (with the defeat of Steppenwolf), the ring would naturally find its way to Hal Jordan to set up the next film.
3 INTRODUCING OTHER DC WORLDS
Another scene that appeared in the trailer that was quickly dissected by fans online was the flashback sequence that took place during Steppenwolf's original attempt to conquer the Earth thousands of years ago. He was fought off by a shocking alliance of the Gods of Earth, the Amazons, the citizens of Atlantis and even a Green Lantern!
Since the flashback was packed with so many different characters, fans quickly guessed that it could be used to introduce a number of different established characters.
One popular theory was that it would introduce the Thanagarians to the DC Extended Universe. This could then be used to set up the introduction of Hawkman and Hawkwoman by the end of the film when Steppenwolf returns to Earth, perhaps as representatives from Thanagar.
2 ALTERNATE REALITIES
The set up of Injustice: Gods Among Us is not just that Superman goes crazy and the heroes of Earth have to choose sides between Batman's rebellion and Superman's tyranny. The big twist that set the game's plot going forward is that Batman brings in the DC superheroes from the normal DC Universe to help defeat Superman.
A number of fans theorized that the shot from the trailer where it looks like the same scene just with different backgrounds suggested that they were alternate realities. Of course, now we know the tonal shift was thanks to the effects of Steppenwolf changing the Earth after he got a hold of the three Mother Boxes. Still, the Multiverse is a big part of the DC Universe and it would have been interesting if it had worked into the DC Extended Universe, as well.
1 SHAZAM DEBUTING
In the aforementioned flashback sequence, where the people and gods of Earth unite to defeat Steppenwolf on his first journey to Earth, it would have been a logical place for Shazam, the famous wizard who gave Captain Marvel (now called Shazam, himself) his powers, to help defend the planet.
Therefore, a cool fan theory was that the Wizard would appear in the flashback and, like the aforementioned bit about the Thanagarians possibly being featured in the flashback, it would set up the introduction of the superhero Shazam by the end of the film. Perhaps the Wizard would actually give Billy Batson his powers specifically to help defeat Steppenwolf alongside the heroes of the Justice League. This could then lead into the already announced future Shazam! solo film. Alas, this path was not the one chosen.