A cool plot could be that the magic - based characters from Arrowverse, like Constantine, cross paths with the magic - users in the Justice League universe, such as Zatanna. They discover a dark magic that is spreading across both universes and must combine their powers and knowledge to stop it. This would involve a lot of magical battles and exploration of different magical realms.
Perhaps in the fanfiction, the Green Arrow gets recruited by the Justice League to help them with a stealth mission. Since he's an expert in archery and has great combat skills in close - quarters, he could prove to be a valuable asset. The story could explore how he adapts to working with super - powered individuals like Superman and Wonder Woman.
One idea could be that the Flash from Arrowverse accidentally runs into a different dimension where he meets the Justice League. He then has to team up with them to stop a powerful villain that is threatening both his world and the Justice League's world.
One idea could be that Robin is on a solo mission when he accidentally stumbles into a Justice League operation. He then has to prove himself to the League members and ends up joining them for that particular mission. Another could be Robin discovers a secret about a member of the Justice League that puts him in a difficult position of whether to expose it or not.
One idea could be Sonic's speed causing chaos among the Justice League at first. For example, he might accidentally disrupt Batman's plans during a Gotham operation. Another could be Sonic teaming up with The Flash to outrun a common enemy that is threatening both their worlds. Maybe they have a friendly race while also trying to lead the bad guy into a trap.
Another interesting interaction would be between Oliver Queen (Green Arrow) and Batman. Both are non - superpowered heroes who rely on their wits, combat skills, and gadgets. They could share their knowledge of different fighting techniques and strategies. Oliver could learn from Batman's more high - tech approach, while Batman could gain some insights from Oliver's more improvised and outdoorsy combat style.
A possible plot could involve a member of the Batfamily getting kidnapped and the Justice League helping to rescue them. During the rescue, they discover a secret weapon or technology that the villains were planning to use against the world, and they have to work together to stop it.
One idea could be a crossover where characters from Arrowverse and Young Justice have to team up to stop a multiverse - threatening villain. For example, Green Arrow and Nightwing leading the combined forces.
One idea could be a crossover where the Young Justice team has to team up with the Justice League to face an interdimensional threat. Maybe a powerful villain from another dimension is trying to invade Earth and the combined skills and powers of both groups are needed to stop it.
Maybe a story where some of the younger members of the Young Justice team are frustrated with the Justice League's methods and decide to take on a mission on their own, but end up getting in over their heads and have to call in the League for help. This could lead to a lot of character development and some interesting dynamics between the two groups.
One idea could be that the Justice League members gain new gaming - related powers. For example, Superman could have the ability to instantly analyze any opponent's weaknesses like a game character stats screen. Batman could use his detective skills to hack into virtual reality systems within the story's universe.
One idea could be a story where Batman has to lead the Justice League in a fight against a new, powerful and mysterious villain that can manipulate time. Another could be a fanfiction exploring the relationships within the Justice League when they are stranded on an uninhabited planet and have to rely on each other more than ever.
Perhaps in the fanfiction, there is a parallel universe where the Justice League are all vampires. They are trying to protect both the vampire and human populations from a greater evil that threatens to destroy both. They face challenges in getting the human - dominated Justice League in our universe to trust them and work together.