The Legion of Doom have been the villains of Legends this year but I'm convinced the Arrowverse is slowly building towards the Justice League, given they've basically introduced the Hall of Justice and Gideon almost mentioned it in The Flash 1x20. If so I think next years' 3 part crossover, well 4 if the 20 seconds they include in Supergirl counts lol, would be the best time to introduce it, and I think their enemy this time should be the Legion of Doom. Only trouble is most of the current Legion's members are time displaced characters so most will be returned to their times by the end of the season, or killed, so I think its membership should be altered so with more present day villains if they appeared in the crossover. There would also need to be more than a few villains because with the way the series is going if the Justice League formed next year it would most likley be comprised of Oliver, Barry, Sara, Kara, Diggle, Ray, Nate, Cisco, Jax + Stein and J'onn J'onzz, maybe even Curtis and James Olsen, provided Supergirl even remembers him?

If it were up to me I’d have the present day Legion of Doom consist of the following:

  • Vandal Savage

    Since he doesn't die until 2166, Savage could still recur in the Arrowverse.

    Vandal Savage - Savage has been depicted as the leader of the Legion of Doom in other forms of media whenever Lex Luthor isn't, and since the Arrowverse's Savage doesn't die until 2166, I think he can still make appearances in the present day of the other shows. Yes he was killed in 1958, 1975 and 2021, though the last one was the 2166 version, but that doesn't mean he's erased because his previous actions still remain intact. Also Reverse-Flash is erased from existance yet that doesn't stop him ruining Barry's life every other week. Also Malcolm does possess 2015 Savage's ashes and Malcolm is currently in the Legion of Doom itself so maybe he'll bring Savage back in some season finale plot twist? I know Savage's history is mostly with Hawkman and Hawkgirl and they weren't the best developed characters, but I quite liked Savage and I don't think the Hawks need to appear for Savage to appear. In the comics his archenemy is Immortal Man yet he didn't always appear whenever Savage appeared. 
  • Deathstroke

    An absolute must add, Slade has been benched for way too long when there are so many storylines his character could have.

    Slade Wilson/Deathstroke - Without question if there is a present day Legion of Doom then Slade absoutley must join, Mirakuru or not. Slade is still considered Arrow's greatest villain and as his character is a mercenary who followsthe highest bidder, and is completely the type to follow the winning side I could see him joining Vandal Savage if for no other reason than to kill Oliver. If he has his Mirakuru back I could also very well see him fighting Barry as he has enhanced strength and senses and is probably as quick and strong as Grodd. He also does know Sara and it'd be funny to see him and interact with Cisco and Caitlin as he almost killed them in 2x19. There are rumors surfacing that Manu Bennet could be returning to Arrow this season and I hope they're true, and Stephen Amell also tweeted the Deathstroke mask on the Lian Yu beach, so hints perhaps? If he does I hope he becomes a more recurring antagonist in the Arrowverse which would make this possible as he is one of DC's strongest villains and deserves far more attention than he's recieved in the last few years. 
  • Dark Archer

    Since he already is a member of the Legion, and generally follows every villain orgaisation, it'd make sense if Malcolm joined again.

    Malcolm Merlyn/Dark Archer - Since Malcolm is already a member of the actual Legion of Doom and since he always follows whatever villainous organisation, League of Assassins, H.I.V.E. etc is trying to destroy the heroes, makes sense he'd join the Legion again if given the chance. Also if Slade is invovled as stated above that could be interesting to see him and Malcolm interacting as he murdered Moira who I still think Malcolm, in his twisted Malcolm-way, did still love. I have also been praying to see a Dark Archer vs Deathstroke match, no doubt through in-fighting, as they are my favorite Arrowverse villains. Also as stated above if Savage is invovled Malcolm will probably be the reason why as he'll probably bring him back to torment the Legends in retribution for foiling his attempts to alter his past, as no doubt will be the end result of LOT Season 2. 
  • Killer Frost vs Vibe

    If Caitlin does become evil her being part of the Legion would create tension among Team Flash, given what she means to them.

    Caitlin Snow/Killer Frost - Provided Caitlin ever actually does become Killer Frost, this one is also a good choice. Given what she means to Team Flash they might hesitate to fight her, trying to find her innocence again. However since Team Arrow doesn't interact with Caitlin as much they might be more willing to fight her which could create some tension between the hero teams. Also as Professor Stein is fond of her this might cause some division within the Legends as well. If Killer Frost is more spit-personality than straight up evil like her Earth-2 counterpart, it could also be interesting if her actual Caitlin self struggling to stop her evil half. Also I think we can agree that a fight between Firestorm and Killer Frost is well over due as they are traditionally archenemies. I would perfer if Ronnie somehow came back to be Firestorm fighting his now villainous wife, as I still don't tihnk he's dead, but I'd settle for Jax too. Also how tragic would it be to see Killer Frost vs Vibe?
  • Black Siren talks to Green Arrow

    Similarly Earth-2 Laurel would create tension, given she looks like Earth-1 Laurel and Oliver is still trying to prove she isn't beyond redemption.

    Laurel Lance/Black Siren - For similair reasons to Caitlin, adding Earth-2 Laurel could also cause some tensions between Team Arrow and Team Flash as Oliver is hell bent on proving that Laurel-2 isn't beyond saving, though Barry is more level headed and knows Laurel-2 is a murderer. However since she is also Laurel's doppelganger this could cause further tension within the Legends and test Sara's jugdement as captain, provided she still is captain by then, if she has the same reaction that Oliver did. I'm hoping as much as everyone that Laurel-2 reforms at some point but I also know that until then she is a villain and she'll probably be one for some time. This could be a good time to start it if Laurel joins the Legion for self-preservation but then helps the League after realizing that Savage's plans will get them all killed, though insists she isn't a hero only to save Sara and Oliver in a moment of confliction while haunted by memories of their Earth-2 counterparts.
  • Captain Cold

    While Killer Frost could substitute for his cold gun, Snart is still a criminal mastermind. Also if it is the younger time displaced version in Legends right now, that'd be tricky to work around.

    Leonard Snart/Captain Cold - Like Malcolm, it'd make sense of Snart stayed on with the Legion of Doom if they reformed again in the present day. Given the recent episode of Legends I think this would make sense. After all who says Snart is going back to his time when Legends finishes? He may stick around in the other shows for a while allowing us the have the cool douchebag version of him again but he'll eventually be sent back to restore the timeline. While the Legion would have Killer Frost whereas Snart simply uses a cold gun she isn't a master stratagist and theif like he is and again he has sentimental value to the Legends and Barry. The fact that he is time displaced would also present problems as they simply couldn't kill him without endangering the Legends past, as Snart dying before his actual death means Ray or Mick, as seen LOT 1x15, will die instead but that may also happen if they leave him running around in the present for too long. Also Captain Cold is simply too cool, pun intended, to not have stick around in the Arrowverse more often. 
  • Cyborg Superman

    The Legion would need at least one villain who could fight Supergirl.

    Hank Henshaw/Cyborg Superman - As skilled as every Earth-1 Arrowverse superhero is, Earth-38's Supergirl is stronger than all of them combined and could easily take out the entire Legion single handed so there would need to be someone who could physically fight her. My vote would be for Hank Henshaw to somehow get marooned on Earth-1 and be extorted into helping Savage in exchange for a way back to Earth-38. As I think Martian Manhunter would be invovled if there is another crossover that included the Justice League, this would allow Cyborg Superman to appear as well and I think Henshaw could fight Supergirl and Martian Manhunter at the same time. 
  • Clock King

    I liked this guy and a present day Legion of Doom would need their own anti-Felicity.

    William Tockman/Clock King - Now this may seem like a strange choice but if the Justice League is coming then no doubt Felicity would be their stand-in Watchtower, as demonstrated by Tess Mercer in Smallville, and the fact that Felicity's codename is Overwatch is a clear hint of this. If so then the Legion would probably need their own tech-nerd to keep Felicity off their trail. Now other candidates could be The Calculator or Bug-Eyed Bandit but I highly doubt Felicity's father would help a gang of lunatics, when he went out of his way to protect her from one last year and Bug-Eyed Bandit is comatose. Also adding Tockman with Snart and no doubt Mick, hero or villain, would be invovled then that would give us a small Prison Break reunion. Now all that's missing is Werner Zytle / Vertigo though I don't see how he could fit in, and adding too many villains would be a problem. 
Speedy, Green Arrow, Black Canary, The Flash, Spartan, Hawkgirl and Carter Hall after facing Vandal Savage

No doubt Savage would be pissed about this, and the fact that he was killed by 7 heroes working against him may give him the idea to seek out the Legion.

As for plot it would probably invovle Savage being out for revenge for his previous death at Oliver and Barry's hands and preventing him from killing Hawkman and Hawkgirl but since facing the heroes alone is what got him killed, he recruits the Arrowverse's most lethal villains to even the odds. He would also desire to keep the promise he made Oliver and Barry and try to destroy both their cities by building a bomb right between both cities, ranging around 600 miles, to vaporize both cities which would kill everything in between. However there would no doubt be some in-fighting among the Legion, because in every form of DC media villain team ups always lead to at least one screwing another one over.

I don't have much else to go on besides this and I don't really know how it would work but I think if the Justice League is coming into the Arrowverse eventually then the Legion of Doom should be their enemies in the next crossover and this is my ideal present day Legion of Doom. Tell me what you think or what your ideal Arrowverse Legion of Doom is.