Maybe the Flash could visit the MCU. He could race against Quicksilver. It would be a really cool showdown of speedsters from different universes. And then the Flash could get involved in an MCU - level threat, like Thanos' invasion but in a different timeline. He could use his speed to try and steal the Infinity Gauntlet from Thanos, creating a whole new plotline for a fanfiction.
For a more complex crossover, imagine that the magical elements of the Arrowverse, like Constantine's magic, cross over to the MCU. Doctor Strange and Constantine could team up to close a rift between the two universes that is allowing dark forces to seep through. They would have to combine their different forms of magic and knowledge, and along the way, they'd meet other characters from both universes and have to convince them to work together.
One idea could be a story where characters from different Arrowverse shows team up to fight a new, powerful villain from a different dimension. For example, Oliver Queen from 'Arrow', Barry Allen from 'The Flash', and Kara Danvers from 'Supergirl' could combine their skills and powers to take on this threat.
A cool concept could be a crossover where the technology in Gotham and the magic in Star City somehow get mixed up. Batwoman, being more used to high - tech gadgets, and Arrow, who has some experience with magic - related threats, have to combine their skills to set things right. For example, a device that was supposed to enhance Batwoman's suit ends up being enchanted by magic from Star City, and they need to figure out how to fix it and stop the villain who caused the mix - up.
One idea could be that the Winchesters end up in Star City and team up with the Arrow to fight a demon that is using technology to cause chaos. Maybe the demon has possessed some of the tech - savvy villains in the Arrowverse.
One idea could be that a Sith Lord somehow gets transported to the MCU universe and tries to take over Hydra. They could use their dark side powers to manipulate the already corrupt organization. Another could be a story where a young Jedi in training from the Star Wars universe ends up in the MCU and teams up with the Avengers to fight a Sith who has also made their way there. Maybe they have to combine the Force with the superpowers of the Avengers to defeat the Sith.
A great plot could involve Spider - Man from the MCU getting lost in the DCAU and teaming up with the Teen Titans. He could teach them some of his acrobatic fighting moves while they could introduce him to some of their more out - there villains. This would be a fun way to combine the more light - hearted Teen Titans with the street - level heroics of Spider - Man.
Another way is through shared villains. Say a villain has the ability to travel between the two universes and is causing chaos in both. This would force the heroes to come together. For instance, Loki could find a way to the Arrowverse and start causing trouble. The Arrowverse heroes would be out of their depth dealing with a god of mischief, so they call in the Avengers for help. The two groups would have to learn about each other's abilities and strategies to take on Loki.
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.
Well, in winx mcu crossover fanfiction, a cool idea is the fusion of magic and superpowers. The Winx's magical transformations could be compared to the Hulk's transformation, in terms of a physical and power - based change. Also, the idea of the Winx attending a school like the Xavier Institute in the MCU could be really fun. They could learn about both their magic and how to coexist with mutants and superheroes.
One idea could be a crossover of characters from different Arrowverse shows on Earth X. For example, having Barry Allen from 'The Flash' team up with Kara Danvers from 'Supergirl' to fight against the evil regime on Earth X. They could use their super - speed and super - strength respectively to take on the powerful foes there.
One idea could be creating a male OC who is a brilliant scientist from a small town. He discovers a new form of energy that attracts the attention of Tony Stark. Another could be a male OC with super speed who has to hide his powers until he's forced to use them to save a city. Maybe a male OC who is an alien prince stranded on Earth and has to learn about human culture while also trying to find a way back home.