ByMichael Patterson, writer at Creators.co
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Michael Patterson

To say that we're excited for the CW's upcoming four-show superhero crossover would be an understatement. Ever since we found out that Supergirl would be moving to the CW, anticipation was high and hope was held out for a potential crossover with other Greg Berlanti-helmed superhero shows. Well, our wish was granted, and we now know that Supergirl will cross over with The Flash, Arrow and Legends Of Tomorrow, in a four-night, four-show crossover extravaganza.

Berlanti recently told EW that they have indeed started work on the upcoming crossover and revealed a few details about what might set the events in motion. He said:

"I’m going to probably try to have one unifying threat, as opposed to different things on different ones, which is what the first year was."

So, in order to tie each show together, Berlanti will have one overarching nemesis that will seemingly bring the heroes together. This sounds exciting, nothing better than a solid threat to lead to a superhero team up. But the question is: Who could that threat be?

Legion Of Doom

Villains:  The 'Legends of Tomorrow' Legion of Doom.
Villains: The 'Legends of Tomorrow' Legion of Doom.

What better threat to initiate a superhero team up than a villainous team up? It was recently revealed that the iconic Legion of Doom would be coming to DC's Legends Of Tomorrow. That means we'll have a somewhat different Captain Cold, returning to his rogue-like ways and joining forces with Malcolm Merlyn, Damien Darhk and Reverse-Flash. And since Matt Letscher who plays Eobard Thawne/Reverse-Flash has now been added to Legends as a series regular, expect to see him and the team pop up quite often. With a time traveler (and Earth-traveler), a magician, an evil mystical immortal and one of the most cunning rogues we know, this supervillain team looks damn near unstoppable.

Speaking of Malcolm Merlyn, perhaps he could go solo again and become the major threat. I know that the resident Dark Archer isn't super-powered and thus isn't able to travel to other Earths, but he is capable of shocking us again and again. Remember, he is after all "the Magician" and is known to do the impossible — he did come back to life, become Ra's Al Guhl and constantly appear and disappear out of nowhere. But perhaps the most important reason why it could be Malcolm Merlyn is John Barrowman's new contract. Remember Barrowman's recently signed deal will bill him as a series regular of the entire Arrow-verse and thus, will allow him to appear across all four shows. Could that play into effect here?

Dark Archer: John Barrowman as Malcolm Merlyn.
Dark Archer: John Barrowman as Malcolm Merlyn.

Furthermore, let's not forget Wentworth Miller was the first to sign one of these deals too. So it's highly possible that Captain Cold could be showing up on all four shows as well.

The Legion of Doom is one of the most formidable collections of villains ever seen in the comics and so it's easy to be excited to for their arrival on the small screen. While they will undoubtedly be a force to be reckoned with for the Legends, they would be adversaries worthy of a Flash, Green Arrow and Supergirl team-up as well — remember three of these characters were former Big Bads of either The Flash or Arrow. And with Barrowman and Miller's new series regular deals and Matt Letscher's new regular role on Legends Of Tomorrow as well as his original recurring role on The Flash, expect to see the Legion of Doom popping up all over the Arrow-verse.

The Inhabitant Of The Pod

Supergirl sees the mysterious new arrival.
Supergirl sees the mysterious new arrival.

We have learned that Supergirl herself will likely be the one to initiate the crossover. After all, Supergirl will kick off the crossover week, airing on Monday night and the rest will follow. This will be intriguing as we already know that Supergirl takes place on a different Earth than the rest of the Arrow-verse. So the question remains: What on Earth (1 or 2) could make Supergirl venture into the Arrow-verse? But perhaps that's just it: The threat might not be of this Earth. It could very well be the being that is in the pod that crash landed at the end of Supergirl Season 1. We know that Chris Wood will be portraying the invader, but we are still unsure of his identity.

If the inhabitant of the pod turns out to be evil, perhaps Supergirl will need the help of the rest of the heroes from the Arrow-verse. She already knows Barry; he's just an Earth away.

Mirror Master

Mirror,  mirror on the wall: Mirror Master.
Mirror, mirror on the wall: Mirror Master.

One villain that has been noticeably absent from The Flash is comic book archenemy Mirror Master. Despite being one of the most notorious Flash villains, Mirror Master has never appeared on the CW TV series. But perhaps what makes him a likely threat is that some iterations of the character could actually use mirrors as portals, moving in and out of the "mirror-verse." Somebody has to be able to travel between Supergirl's Earth and the Arrow-verse, and if it's not Barry's speed that does it, perhaps the mirror portals is the way to do it.

Traveling between Earths through mirrors? Stranger things have happened on the shows before, and stranger things will happen again. If this could mark the debut of Mirror Master and bring us the highly anticipated crossover we've all been dreaming about, them I am all for it!

Gorilla Grodd

Grodd Lives: Grodd meets the Flash.
Grodd Lives: Grodd meets the Flash.

When we last saw Grodd, he had just landed in Earth-2 in what looked like Gorilla City. With him safely out of everyone's way on Earth-1, that should be it right? Wrong. Grodd is one of the Flash's main villains and he hasn't yet reached his full potential on the show. If Grodd were to reach said potential and take over Gorilla City, there's no telling how powerful he could be. And that includes the possibility of being able to tear down the barriers between Earths.

If he showed up on Supergirl looking for The Flash, Kara would have no choice but to track down Barry and return Grodd to where he came from. A disruption in the space time continuum would certainly alert the Legends too. Simply put: If you're looking for one big unifying threat to unite the superheroes, it doesn't get any bigger than Grodd.

Black Siren

Black Siren will return to "The Flash."
Black Siren will return to "The Flash."

When Black Siren screeched through the streets of Central City during the metapocalypse, fans rejoiced at just how awesome she was. They also reveled in the opportunity to see Katie Cassidy return after her character Laurel Lance/Black Canary was controversially killed off on Arrow. However, thanks to fan support, praise for the character and love for Katie Cassidy we now know that at least one Laurel will be back. Katie Cassidy has signed the same type of deal as Barrowman and Miller which will allow her to appear across all four shows. And as the heroic Black Canary obviously won't be the villain of the crossover, there's a possibility that it could be Black Siren.

The last time we saw her, she was imprisoned in S.T.A.R. Labs, but if she were to get out or the timeline was reset, she could always end up becoming a major threat — remember, she may have been Zoom's lieutenant, but she clearly wanted to run her own operation. Maybe now is her time. If Siren were to somehow show up on Supergirl, this would solidly link it to the rest of the shows. She's the Flash's nemesis, looks like Laurel and would thus, attract Team Arrow and her sister Sara from Legends Of Tomorrow into the equation. Thus, in effect, linking all four shows.

With a strong connection to three shows and the possibility of appearing on the fourth, Black Siren could quite possibly be the ideal choice for the villain of the crossover.

Scream and be free: Siren trapped in S.T.A.R Labs.
Scream and be free: Siren trapped in S.T.A.R Labs.

Well, it's looking like the annual Arrow/The Flash crossover event will be bigger than ever this year. Aside from being awesome as always, it will also mark Arrow's 100th episode, introduce Supergirl to the Arrow-verse, bring in the Legends for reinforcements and have one unifying threat for them all to take down. But who on Earth could be a big enough threat to cause the biggest superhero team up ever seen on television? Time will tell. All we know is that the heroes will be ready.

And don't just take my word for it, here's the confirmation from the Green Arrow himself, Stephen Amell:

Yep, I see Supergirl's red boots too. The heroes are ready, and the crossover is coming!

Are you excited for the upcoming major superhero crossover on the CW? Who do you think the major threat could be? Let us know in the comments!

[Source: Entertainment Weekly]