With another crossover coming next year I think this is a prime chance to offically introduce the Justice League into the Arrowverse because quite frankly, it's well past due. If so I think their enemies should be the Legion of Doom and here is what I would do if I were a show runner.
When Gypsy returns to follow a breacher from Earth-19 Grodd takes control of her and escapes A.R.G.U.S. and kidnaps several criminals including Dark Archer, Deathstroke, Black Siren and Killer Frost. He also forces Gypsy to travel to Earth-38 and captures Hank Henshaw and brings him to Earth-1. Grodd gives the six villains an ultimatum to either obey his orders and be spared from the wrath of his plans, or to die painfully and the six are forced to agree and the Legion of Doom is reassembled. After hearing of the mass A.R.G.U.S. breakout Team Arrow, Team Flash, the Legends and Supergirl combine their forces yet again to find their enemies and stop them.
While it’s assumed that Gypsy is unwillingly under Grodd’s control and the team has no trouble fighting Malcolm, Slade and Henshaw they are less sure about how to handle Caitlin and Laurel-2 given what Caitlin means to them and what Laurel-2’s counterpart meant to them. This also causes Sara discomfort to learn of Earth-2 Laurel’s existence from Oliver and for Stein to learn that Caitlin has succumbed to her Killer Frost side. Ray also becomes vengeful after learning Slade is one of their opponents as he’s the reason Anna died and the reason he became The Atom in the first place. Cisco is equally nervous about possibily having to face Gyspy again. Oliver and Barry speculate that Grodd planned for this and chose criminals who have defeated them, and those that hold sentimental value to them.
Shortly after the heroes are confronted by the Legion of Doom and the odds prove fairly even without Grodd being present. During the fight however they are unexpectedly confronted by A.R.G.U.S. agents as Grodd is revealed to have Lyla under mind control, and has declared them the most wanted terrorists in America. After Gypsy/Grodd breaches the Legion away Cisco desperately makes the same move and breaches them away in pairs around the country. Barry however is captured alive and brought to S.T.A.R. Labs after being forced to fight the entire Legion alone. Meanwhile Grodd has assembled all of S.T.A.R. Labs former staff members, specifically those that worked on the particle accelerator, and forces them to reassemble it with some modifications. While this happens Barry is kept in the pipeline as Grodd wishes for him to see his plans firsthand in retribution for his previous defeats. He is also revealed to have Earth-2 Harrison Wells captive and has been forcing him to aid in rebuilding the accelerator as well.
With the heroes on the run the Legion begins to question why they were even brought in if Grodd simply has A.R.G.U.S. in his back pocket as they come to odds with each other. Malcolm and Slade dislike each over hypocritical indifferences with Malcolm hating Slade for killing Moira and trying to kill Thea and destroying most of Starling City in pursuit of vengeance, while Slade dislikes Malcolm for corrupting Thea and for believing that his Undertaking is somehow more validated than his Siege. He also thinks of himself as better than Malcolm as he trained to be a killer because his wife died and in pursuit of his own vendettas whereas Slade trained his whole life to be a killer in service of his country, then betrayed by those he cared for.
Likewise Caitlin and Laurel-2 come to odds with Laurel-2 regarding Caitlin as a pathetic attention seeker with mental issues, whereas her Earth-2 counterpart was a straight up sociopath who killed because she loved to not because she has an alter-ego making her do it. Caitlin however views Laurel-2 in a similar light, as nothing more than a frightened child screaming out at the world who revels in death because she lost everyone she cared for, and needs to make everyone else feel the way she does. Caitlin also thinks this is why she hates Laurel-1 because despite being dead she lived the life Laurel-2 wanted, one where her family lived. After making sassy comments about boat trips (Earth-2 Oliver) and blackholes (Ronnie) the two come to blows as do Malcolm and Slade, though they are put back in line by Gyspy/Grodd.
Shortly after Grodd gives the Legion their next assignments and orders each Legion member to hunt down a group of the scattered heroes, now that they have been all but backed into a corner, and has Caitlin go after Cisco, Jax and Stein, Laurel-2 goes after Oliver, Sara and Diggle, Slade goes after Ray, Mick and Amaya and Henshaw goes after Kara and Nate. He also gives them each a vile of a chemical to disperse when confronting them. As each Legion member confronts their targets they disperse the chemical which is a new airborne form of the vertigo drug, as Grodd forced Werner Zytle to replicate it, which paralyses each hero into shock as they relived and hallucinate their worst nightmares, while the Legion members beat down the incapacitated heroes.
Meanwhile Wally travels to Starling City to get Team Arrow’s aid and teams up with Dinah, Thea, Rene and Curtis to rescue Barry from Grodd. Upon arriving however they are confronted by Gypsy/Grodd but they manage to overpower her together and rescue Barry and Harry from Grodd. Eventually the Legion members are called back but Oliver manages to capture Laurel-2 as the group reunite onboard the Waverider and Barry reveals what Grodd told him. Grodd has been using the scientists to rebuild the particle accelerator to unleash it on a country wide scale and turn every gorilla in America into a meta-gorilla, but will turn almost everyone in America into meta-humans at least the ones who survive the blast wave. Furthermore Grodd has no way of knowing if it will even work on gorillas as he became who he is with a combination of dark matter and drugs General Eiling exposed him to years before.
With the Legion of Doom in S.T.A.R. Labs and Lyla under Grodd’s control at A.R.G.U.S. the heroes divide into two teams. One is led by Diggle who takes Dinah, Wally, Thea, Curtis and Rene to A.R.G.U.S. to keep Lyla distracted while the rest are led by Oliver who plan to fight the Legion of Doom at S.T.A.R. Labs and stop Grodd’s plan. Unable to attack it head on however and risk Grodd prematurely activating the accelerator they seek Laurel’s help, but she refuses out of self preservation. However Sara pleads to her for help and eventually Laurel, having a fondness for Sara being her sister’s doppelganger, reluctantly agrees. Laurel then returns to S.T.A.R. Labs with a “captive” Cisco, and Ray secretly shrunken and hiding on her shoulder, and once inside Ray attacks as Cisco opens a breach to let Barry in who swiftly sabotages the accelerator.
Cisco then swiftly breaches each Legion member to different area around S.T.A.R. Labs, where several heroes are waiting for them which includes Laurel once Grodd forces her back under mind control. While Grodd tries to have Lyla send an A.R.G.U.S. patrol Diggle’s team attacks the facility where they face Gypsy again, and A.R.G.U.S. becomes too distracted to respond to Grodd. Cisco and Firestorm defeat Killer Frost after failing to reason with her while Oliver and Sara fight Malcolm and are almost defeated until Laurel, while fighting Amaya, impulsively saves them, Ray and Nate defeat Slade while Kara defeats Henshaw and in further assurances sabotages the accelerator completely. Eventually Grodd is defeated by Barry with assistance from Oliver and Sara and Cisco breaches him back to Earth-2, right into Gorilla City where Solovar is waiting to exact vengeance for his previous defeat.
In the aftermath the Legion manage to escape minus Henshaw who is taken back to Earth-38 by Kara and Caitlin who is locked up in the S.T.A.R. Labs pipeline, where Barry and Cisco hope they can help her find her innocence again. Laurel escapes but is left more conflicted with prolonged interaction with Oliver and Sara as they are more hopeful she isn’t beyond saving. Lyla calls off the manhunt for the heroes while Gypsy returns to Earth-19 after she catches the breacher she was initially after, while Harry also returns to Earth-2. Both Oliver and Diggle’s teams regroup at the S.T.A.R. Labs hanger where they are all praised yet again by the President for averting a potentially extinction level event, even under the threats their own government put them under. Afterward Cisco decides that with the amount of times they keep teaming up they really need a team name and as Cisco, Ray and Nate think through dozens of possible group names, much to Oliver’s annoyance, Sara comes up with the group name “Justice League”. Cisco then jokingly mentions calling the S.T.A.R. Labs hanger "Hall of Justice" but Oliver dismisses it as lame.
I think we are well overdue the offical Justice League introduction because come one why wouldn't they? And I think having them face the Legion of Doom would be the best way to make it offical. Tell me what you think if you liked it and thanks for reading.