Well, a great concept for an Arrow - Avengers crossover fanfiction would be a race against time. A powerful and evil force is about to destroy both universes. The Avengers and Arrow team have to work together. Black Widow and Speedy could team up and use their acrobatic and combat skills to infiltrate the enemy's base. Thor could help Oliver with his new - found knowledge of magic arrows. And the Hulk could be the brute force that the Arrow team sometimes lacks in their battles against meta - humans.
One idea could be that Oliver Queen gets transported to the Avengers' universe. He could teach them some of his stealth and archery skills. Maybe he helps Hawkeye improve his arrow techniques. Another idea is that the Avengers come to Star City. They help Oliver fight a new, super - powered villain that is too much for him to handle alone. The Avengers' powers and teamwork could combine with Arrow's street - level fighting style in an interesting way.
One idea could be that Oliver Queen from Arrow gets transported to the Avengers' universe. He could teach them some of his unique archery skills and in return, the Avengers could show him advanced technology like Stark's Iron Man suits. Maybe they team up to fight a common enemy that has the ability to manipulate time, causing chaos in both their worlds.
One idea could be to explore a world where the characters from 'Lucifer' and 'Arrow' are forced to team up to stop a common supernatural threat that is beyond the capabilities of either group alone. For example, a powerful demon that is using advanced technology to cause chaos in both their worlds.
One idea could be Batman coming to Star City to help Arrow deal with a new, extremely dangerous criminal organization that has ties to Gotham. This could lead to some great fight scenes as they combine their skills and resources.
One possible plot could be that Oliver Queen gets transported to the Avengers' universe. He teams up with Hawkeye, sharing their archery skills and combat tactics. They could go on a mission to stop a new threat that combines the technology of the Arrowverse and the super - powered enemies of the Avengers.
One idea could be that the Flash and Arrow team up to stop a meta - human who has the ability to control time. They have to race against the clock (literally) to figure out how to neutralize the villain's powers before he rewrites history in a way that destroys Central City and Starling City.
Well, a great plot for Supergirl Arrow crossover fanfiction could be centered around a common enemy from another dimension. This enemy has technology that nullifies Supergirl's powers and Arrow's skills, so they have to rely on their wits and each other's unique strengths. For example, Supergirl could use her heat vision in a creative way to expose the enemy's weaknesses while Arrow uses his archery skills to set up distractions. They could also meet some interesting characters from the other dimension during their adventure.
One idea could be that Batman and Arrow team up to take on a common enemy like Ra's al Ghul. Since both of them have had run - ins with him in their respective storylines, it would be cool to see them combine their skills and resources to fight against him. Another plot could involve a mystery that spans both Gotham and Star City. Maybe there's a new criminal organization that's using technology from both cities to carry out their evil plans, and Batman and Arrow have to figure out how to stop them while also dealing with the differences in their methods of crime - fighting.
One idea could be that an alien threat emerges which requires the skills of both Arrow's team in Star City and Supergirl in National City. They have to combine their combat styles, with Arrow's more grounded and stealthy approach and Supergirl's super - powered attacks. For example, Arrow could use his trick arrows to create distractions while Supergirl takes on the stronger alien foes directly.
A great plot could involve the two heroes swapping cities for a while. Arrow tries to deal with Gotham's crime underworld while Batman goes to Starling City to investigate a series of strange disappearances that might be linked to a powerful criminal organization there. This would lead to both heroes having to adapt to different environments and crime - fighting styles.
One idea could be that the characters from Torchwood and Arrow team up to fight a common enemy that has some sort of interdimensional powers. Maybe a villain from Arrow's world has found a way to access Torchwood's technology and is using it for evil, and the two groups have to combine their skills to stop it.