With Arrow's 3rd season concluded, Flash's 1st season only one episode away from finish and with this new Legends of Tomorrow thing on it's way, CW should brave their own Justice League tv event which would likely be a crossover between an episode of Flash and Arrow, or maybe even a be a TV movie and here’s how think it should go:
The justice league itself should consist of:
- Oliver Queen / Green Arrow
- Laurel Lance / Black Canary
- Barry Allen / The Flash
- Ray Palmer / The Atom
- Ronnie Raymond / Firestorm
- Professor Martin Stein / Firestorm
- Tatsu Yamashiro / Katana
- Kendra Saunders / Hawkgirl (from the upcoming "Legends of Tomorrow" series)
Plus Diggle, Felicity, Cisco and Caitlin could also appear to help the team to support with technical issues.
If Supergirl is part of this share universe, maybe her but it isn't confirmed as far as I know
The villain should be arguably the Arrowverse’s most iconic villain Slade Wilson / Deathstroke who would be powered by Mirakuru again but this time has modified its properties so the original cure won’t work on him. In this story he should forced to work for Amanda Waller as a member of the Suicide Squad but after months of extorted work manages to get free, and recreates the Mirakuru for himself and begins a plan to destroy every city in the world connected to A.R.G.U.S. in retaliation for imprisoning him with the intent of decimating A.R.G.U.S. itself before killing Waller herself but at the cost of hundreds of innocent lives. To do this he travels to Nanda Parbat to challenge Malcolm Merlyn to a duel to assume command of his army and manages to force him to hand over his army using Thea as leverage. This would also allow a Dark Archer vs. Deathstroke match. After assuming command of Malcolm’s army, after imprisoning Malcolm and Thea on Lian Yu, he plans to use it to wipe A.R.G.U.S. off the global map starting with Hong Kong using some kind of super-weapon.
Team Arrow and Team Flash would be resuming their hero activities in Starling and Central but find activity of the league of assassins in both their cities stealing weapons, and both teams follow up on it and join forces when they discover they’re following the same enemy. When Oliver, Laurel and Barry discover they're hunting the same enemy they unite their forces and call in Ray and Ronnie/Stein for back up. They also encounter Kendra (Hawkgirl) who after a heated fight joins them, and Tatsu also joins them after learning of the league's world scale threat.
The main location they would base at should be S.T.A.R. Labs, it has more resources and space than the Arrowcave. Of course there would have to be some tension between the heroes themselves as some of them have never met each other and would have different views on how to handle justice. Eventually once the team is established they are told of the threat of Slade from Waller but in preparation for this Slade gathers the Rouges, Captain Cold, Golden Glider, Heat Wave, Weather Wizard, Rainbow Raider and maybe Captain Boomerang to distract them while he preps his plan.
In the process Rainbow Raider also gets to some of them and they wind up fighting each other in various fights: I'd vote to see Green Arrow vs. Firestorm (so both Amell cousins can fight each other), Black Canary vs. Katana, Green Arrow vs. Black Canary or Atom vs. Firestorm. After resolving these differences and coming to an understanding with each other they track Slade to Hong Kong where he plans to unleash his weapon, where he has the league lock down the city. However the superheroes directly confront the league and engage in a large epic fight with them before advancing on Slade himself, and they’d take turns in fighting him one-on-one, then in teams and eventually all together and eventually defeat Slade, then Malcolm appears and finishes him off with a new cure arrow designed specifically for him, then Thea knocks him out in anger for murdering Moira. The Rouges are imprisoned on Lian Yu minus Lisa who manages to escape and the team decide to part ways but before they do they have a new facility built in the even they need to team up again and they call it the Watchtower and Cisco names their group the “Justice League”.
This is just how I'd have a Justice League event between these tv shows, but leave comments and let me know what you think.