A long time ago, a dark television show called Arrow -- a gritty adaptation of the Green Arrow -- began airing on the CW. Soon it introduced a young CSI named Barry Allen, who shortly thereafter got superpowers. Then more heroes began to show up, then more villains. Soon, shows that seemed unconnected began to connect -- Constantine and Supergirl folding in to join the Universe in which the Arrow fought and the Flash sped around. Shortly after Arrow began, an entire Universe was unfolding, with unexplored gaps, and an almost infinite amount of worlds and potential. It was called the Arrowverse (or, occasionally, the Flarrowverse).

RELATED: These DCEU Fan Theories Will BLOW YOUR DAMN MIND

The Arrowverse is one of the most popular media universes around. Marvel has its MCU, but DC has a television show one that is arguably cooler. It has Vixen, Legends of Tomorrow, Constantine, Arrow, the Flash, Supergirl -- and possibly more coming soon. As it is so understandably cool, it has dozens of fan theories about literally everything that could possibly happen. Because fan theories are the best -- and quite often end up actually happening -- we decided to take a look at some of the most popular, most likely, or just plain coolest fan theories out there for the Arrowverse.

15 ORACLE

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Felicity is basically everyone's favorite character in the entire Arrowverse. She's the genius who helps Arrow find the baddies, she can hack anything, and oh yeah, for a little while she was in a wheelchair. Remind you of anyone? Yep, that's right, she sounds almost identical to Oracle, the alter ego that Barbara Gordon (AKA Batgirl) took up after her tragic shooting and paralysis caused by the Joker.

One theory goes that eventually Felicity will earn the title Oracle and become the character, more or less. In addition to Batman, the Oracle worked with almost every single superhero out there, so for this theory to come true, Felicity is going to need to expand her reach and start helping out the Legends, Flash, and more. We're down for that, because c'mon, who doesn't want more Felicity?

14 CRISIS

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One thing that's been coming -- that we've known has been coming -- since the earliest episodes of The Flash is a Crisis. Back during the early days of DC, the continuity was... well, it was a mess. It was cleaned up with something called Crisis on Infinite Earths, which is exactly what it sounds like. The Crisis began with Red Skies, which is exactly what is referenced in the article about the disappearance of Barry Allen, years in the future, at the end of a crisis. Coincidentally, that is exactly what happens in the comic, Crisis on Infinite Earths.

So, this theory goes, when Arrow, Flash, and the rest reach their end, in maybe seven years, there will come a Crisis, one that is multiversal in nature and will end the entire series in a huge cataclysm, taking the life of Barry Allen. We'll just have to wait and see.

13 GOTH CROSSOVER

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If this doesn't happen, we'll strike. Gotham is one of the weirdest and coolest shows on television, but as of yet it has absolutely no connection to the Arrowverse. Sure, Batman is in it (albeit a young version) and the Arrowverse has mentioned WayneTech, but the shows are on competing networks (like The Flash and Supergirl were) and it seems unlikely that this will happen. But, as always, there is a chance, however slight.

This theory posits that at some point, the kookily weird show Gotham will have a crossover with the Arrowverse and -- this part is just fact -- it will be the most glorious thing to behold. It will be more brilliant than seven exploding suns. It will be perfection and we shall all bow before its glory.

12 SECRET SIX

Secret Six

The Secret Six are some of the most beloved villains in all of comic books. The series was about six supervillains teaming up with each other. It was sort of like Suicide Squad except in Secret Six, the group wasn't controlled by Amanda Waller, and was much, much more fun (surprising as that may seem). There's a prominent theory out there that the Secret Six will turn up in the Arrowverse at one point.

After all, the Suicide Squad is gone now, so some league of villains has to step in and take their place, right? The Secret Six would fit in perfectly, maybe even spinning off into their own show? How awesome would it be to have a Legends of Tomorrow-esque show, but filled with villains? Pretty darn awesome, that is the correct answer. Just FYI.

11 ANIMAL MAN

Animal Man tapping into the powers of the Red

Okay, we admit, this one is less a fan theory and more of... well, it's just something we want really badly, okay? But come on, Animal Man is one of those weird, zany characters that basically only the DC Universe has and it would be a shame to not see him somewhere in the Arrowverse. His power is to tap into the abilities of any known animal on earth (and for a time, even those in space, which was awesome). And yet, even though he can do amazing things, that's not all there is to him. He's also an Avatar of the Red -- the life-source of all things that are made of meat.

At one point, legendarily weird comic scribe Grant Morrison wrote an extremely trippy, fourth-wall breaking series starring Animal Man, in which Morrison and Animal Man met, and Animal Man stared at the reader in horror and exclaimed that he could see you reading the comic. He's had a weird and interesting life and would be the best addition to the Arrowverse since Barry Allen.

10 BIRDS OF PREY

Birds of Prey members Black Canary, Huntress, and Batgirl in DC Comics.

Birds of Prey has already had a show, but that show was extremely bad (or, at very least, an acquired taste). The Birds of Prey comic though is extremely awesome and has a bunch of different female superheroes (and anti-heroes... and supervillains) teaming up together working under the orders (most of the time) of Oracle. The most common members are one of the Batgirls, Black Canary, and Huntress -- and two of them already exist in the Arrowverse!

Adding this cool team to the Arrowverse is so simple and common sense, that we're kind of surprised it hasn't happened yet. One episode featured Black Canary and The Huntress and was called Birds of Prey, but since then, there's been no further development. While this one looks less likely, it's still a popular theory for how freaking dope it would be.

9 BATMAN

Batman in shadows during opening of Animated Series

Batman is, well... Batman is Batman. If you don't know who he is, what the heck are you even doing on this website? Did you stumble here by accident? He's a man dressed like a Bat who punches criminals. He's fire on justice and has zero chill for crime. And there's a solid chance that he's coming to the Arrowverse.

See, the show has been dropping hints about him like Hansel and Gretel dropping breadcrumbs. WayneTech has shown up and Dark Knights have been referenced. People have mentioned vigilantes. Now with Superman having been introduced, there's almost nothing stopping Batman from showing up; it's just a matter of when, where and who. This one is less of a theory and more of a certainty; at some point in the future, there's going to be a Caped Crusader showing up in the Arrowverse and we are so freaking ready.

8 CROSSOVER

Christopher Reeve's Superman at a conference in Superman IV

Okay, so we've mentioned that this universe has a lot of crossovers right? And people desperately hope that it'll crossover with Gotham, but what if it crosses over with things that don't even exist anymore. For instance, the Christopher Reeve Superman movies. That's right, some people think that the Arrowverse will actually crossover with other superhero Universes -- maybe having Brandon Routh as both the Atom and Superman. There might even be a huge crossover with the DCEU -- the films in which Ben Affleck and Henry Cavill are starring as the Caped Crusader and the Big Blue Boy Scout, respectively.

Who knows? These shows have already had interdimensional crossovers; why not a couple more? We know it's not completely out of the question either as the show cast John Wesley Shipp, the actor who previously played the Flash in the '90s, along with some other legacy player cameos. Even if the show doesn't go full-tilt into the Nolanverse, there's a chance for more fun notes like this in the future.

7 DOOMSDAY

11 Superman and Doomsday

Doomsday is probably coming to the Arrowverse, maybe sooner even than we think. Doomsday is, well, honestly his name should give you a hint. He's a nigh unstoppable creature who was responsible for killing Superman in the appropriately titled story, "The Death of Superman." Luckily, Doomsday died in the fight, but he can come back thanks to his unique evolutionarily-reactionary physiology and staggering, all consuming hate. He was created by an alien scientist named Bertron, who basically killed and cloned D-Day until he was an unstoppable brute. He's one of the most fearsome foes in all of the DC Universe and there's a chance that Supergirl just introduced him.

One thing that bolsters this theory heavily is the fact that comics and the television shows seem to work together as much as possible. Coming this Fall -- which is right when Supergirl and the rest will be coming back on -- is a comic series called the Doomsday Clock, starring (potentially) guess who?

6 FLASH TEAM-UP

Flash has met his future self a couple of times but never for more than a few seconds. Occasionally this has led to a time remnant (a weird time travel duplicate that makes no amount of sense), something that came back to bite him in a major way in Season Three of The Flash. SPOILER WARNING: it turned out that Barry Allen's nemesis Savitar was actually a Future Flash. It's a lot to explain if you haven't been keeping up, and if you have, well, then you don't need us!

What if, instead of the nice Flash staying in the future, he had come back to help the younger one? This is the theory that a bunch of people have; that at some point, there will be multiple versions of the same Flash. It's not without precedent either, as the Reverse-Flash pulled the same trick recently. Granted, that did end with his death, but that was because he was a time remnant. Barry will be much safer.

5 ARROW-FORWARDS

Arrow

For all five seasons of Arrow, for better or worse, we've had flashbacks. These flashbacks have slowly revealed what happened to Ollie after he left his home and got stuck on an island, showing the trials he endured, and what happened to make him The Arrow. Of course, it's been five years since that first episode and since he was only on the island for five years, the flashbacks have finally come to an end with this season of the Arrow. So what's next?

What about flash-forwards? Instead of showing how he came to be where he is, what if the entire season shows where he will be? We see what the Arrow is doing in a season finale and then slowly build up to it. It would increase the drama and the sense of suspense. The show has done it before as well, showing a future funeral, that we worked our way to slowly, leaving fans wondering what might happen if it went from future to past. It was a high point of the show and doing that every season could work spectacularly.

4 OPEN OLLIE

Arrow -- "Lian Yu" -- Image AR523a_0609b.jpg -- Pictured (L-R): Deathstroke and Stephen Amell as Oliver Queen/The Green Arrow -- Photo: Jack Rowand/The CW -- © 2017 The CW Network, LLC. All Rights Reserved.

Oliver Queen has been keeping his identity as the Arrow hidden for a long time, even pretending to be a different version of the Arrow after someone else took the fall for him. It's been a long convoluted mess as he eventually became Mayor but what if in season six, Oliver Queen decided to drop the act and come out as a vigilante? There have been hints that he might drop the mask with his more public profile, leaving fans wondering what might happen if he did.

It wouldn't be too weird. After all, most of his villains already know who he is, but the public doesn't. So the only thing that might change is everyone would trust him more. Also, since almost everyone seems to be able to figure out that Ollie is the Green Arrow, it would probably make the show a bit better. Just a little bit.

3 FALLEN TRINITY

DC Trinity DC Comics

What if the reason we've never seen Wonder Woman, Batman, and Superman on the Arrowverse is that they're dead? This is a theory that has been bandied about the internet a lot recently and has actually picked up steam. Sure, Superman has appeared on Supergirl but there's a theory going around that he'll be dying soon (that Doomsday thing again). But we're actually talking about the Universe that the Flash and the Arrow live within.

What if the reason that the Arrow is the first big, public superhero is because the trinity is actually dead? After all, on Legends of Tomorrow, we hear about Men of Steel breaking and Dark Knights falling. What if the only reason all of these admittedly B-list superheroes are the ones saving the Universe is because they're the only superheroes left. And if the Trinity is dead, that raises the question -- what killed them?

2 STARFIRE

Starfire using her starbolts

Starfire is an interesting and cool character from the Teen Titans who might be making an appearance on one the Arrowverse series soon. While this theory is a bit less solid than some of the others, it is really cool and could lead to a bunch of interesting things. If Starfire does show up, it'll most likely be on Supergirl -- which raises the question of where the rest of the Teen Titans are?

See, the Teen Titans are getting their own show and Starfire will be a member of their group. What if Starfire comes to Supergirl as a means to tie those two shows together? What if the Teen Titans are on the same Earth as Supergirl. We've all thought that crossovers between shows on different networks was anathema to them but the Arrowverse has proved that wrong. Could Starfire come to the Arrowverse? With her profile becoming more mainstream again, thanks to her recent animated outings, fans' ferocity for the 'Fire is at an all-time high!

1 JUSTICE

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Right now, the closest thing to the Justice League that the Arrowverse has is the occasional crossovers and the Legends of Tomorrow. But what if the Arrowverse does develop its own, actual League? We've already seen the Justice Society and there are hints of the League potentially coming in the future, but why not now? Heroes coming together to fight a huge threat is something the Arrowverse loves, so why not make a miniseries showing a bunch of them coming together to fight that threat?

It would be like the Invasion crossover, but more permanent, with an entire series devoted to it. Of course, there's an argument that the Legends takes this spot in the Arrowverse, but having an actual League, with Arrow, the Flash, Supergirl, and members from the Legends, plus brand new characters would finally give credence to fans' theories that this is all just prologue to the eventual super series we've all been waiting for!

Do you know any other fan theories about Arrowverse? Fire away in the comments!