You can start by looking on fanfiction platforms like Archive of Our Own (AO3) or FanFiction.net. These sites have a large collection of fanfictions. Just search for 'Arrow and Avengers crossover' and you'll get a list of relevant stories. Some might be really good, while others may not be to your taste. It's all about exploring and finding the ones that resonate with you.
A possible plot could be that Oliver Queen from Arrow gets transported to the Marvel universe where the Avengers operate. He might initially be seen as a threat, but then he teams up with the Avengers to fight a common enemy. Maybe a villain that has powers related to both the technology in Arrow and the superpowers in the Avengers' world.
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 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.
Maybe it could be a time - traveling criminal. This villain has the ability to jump between different timelines and has noticed that by disrupting the key events in both the Arrow and Avengers universes, he can create chaos and then rule over the resulting mess. He would constantly be one step ahead of the heroes as he could manipulate time to his advantage. He could also use his knowledge of future events to set traps for the Avengers and the Arrow team.
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 great 'Supernatural' and 'Arrow' crossover fanfiction is 'The Hunt in Starling City'. It combines the dark, supernatural elements of 'Supernatural' with the gritty, urban setting of 'Arrow'. The story has Dean and Sam Winchester arriving in Starling City to hunt a demon that's been causing chaos in the criminal underworld, which draws the attention of Oliver Queen and his team. They initially clash but then realize they need to work together to stop the greater evil.
One great Flash and Arrow crossover fanfiction is 'The City's Heroes United'. It has a really engaging plot that combines the unique abilities and personalities of the characters from both shows. The author does a great job of making the interactions between Flash and Arrow's characters feel natural.
The plot might involve a villain who has the ability to manipulate time or space. This causes disruptions in both the Marvel Universe and Green Arrow's world. The Avengers, with their superhuman powers and high - tech gadgets, and Green Arrow, with his street - smart skills and archery expertise, must find a way to stop this villain. They might have to travel between their respective universes, which could lead to some really interesting cultural clashes and misunderstandings.
One possible plot could be that the Avengers are somehow transported to the Arrow's universe. Maybe a villain in the Arrowverse has acquired a powerful artifact that creates portals, accidentally pulling in the Avengers. Then, they team up with Arrow and his team to fight against a common enemy, like a new super - powered threat that neither group could handle alone.