One idea could be that a fallen angel joins the Avengers with a mysterious power that can either save or doom the team. For example, the fallen angel might have the power to manipulate time on a small scale, which could be useful in battles but also dangerous if misused.
Another plot could involve the fallen angel bringing a new type of magic or energy into the Avengers' world. This new element might be the key to fighting a powerful, otherworldly threat that the Avengers couldn't handle before. The fallen angel would then have to teach the Avengers how to use this new power while also dealing with their own personal demons.
One idea could be that a fallen angel is sent to the Naruto world to retrieve a powerful artifact that has been lost there. Naruto, being the curious and heroic type, gets involved and they end up having to team up. Another could be that the fallen angel is on the run from some other celestial beings and Naruto offers her protection in his village. A third idea is that the fallen angel has some unique powers that Naruto wants to learn from to become stronger in his fight against the Akatsuki.
A great plot could be that the fallen angel is mistaken for a villain at first. As the story progresses, it's revealed that they are actually trying to stop an ancient evil that is about to be unleashed in the My Hero Academia universe. They have to convince the heroes to trust them despite their appearance and strange powers.
A good plot for Highschool DxD fallen angel OC fanfiction could start with the OC being hunted by a group of vengeful angels. As they run, they end up in the territory of the devils and have to make a deal with them to survive. Along the way, they fall in love with a devil, which creates a whole new set of problems as their two worlds collide. Or, the OC could be trying to solve a mystery that dates back to the very beginning of their fall, something that involves a prophecy that only they can fulfill.
One interesting plot could be a fallen angel trying to regain its former status. For example, it might embark on a series of heroic deeds in the human world to prove its worth to the celestial realm again.
A really cool plot idea could be that the Avengers are watching Endgame in a sort of 'alternate reality cinema' where the events in the movie start to bleed into their real world. So they have to stop the movie from fully playing out to prevent chaos in their own universe. This would involve a lot of action and strategic thinking as they try to stop things like Thanos from emerging from the screen while also trying to understand the significance of what they're seeing in the movie.
Obviously, the fallen angel would be one of the main characters. It could be a character with a tragic backstory, like they were cast out of heaven for trying to protect humanity in a way that was against the celestial rules. The Avengers, such as Iron Man, Captain America, and Thor, would also be main characters as they interact with the fallen angel.
Well, you could make your fallen angel OC a bit of an outcast even among other fallen angels. Maybe they have a different view on the war between angels and devils. For example, they could be trying to broker peace in secret. They might also have a special connection to a certain sacred object that was lost during their fall, and the story could revolve around finding it again. Also, giving them a distinct personality trait like being overly curious about human culture could lead to some fun and unique storylines.
One idea could be Deadpool trying to join the Avengers in his own zany way. He might show up at the Avengers' headquarters with a bunch of crazy gadgets and a self - made resume full of his 'accomplishments' that are half - heroic and half - insane.
Perhaps the Avengers mistake one of the Horsemen for a new super - villain. As they engage in battle, they soon realize that the Horsemen is actually after a powerful artifact that a real villain in the Avengers' world has obtained. So they form an uneasy alliance to retrieve the artifact.
A possible plot is that Elsa's kingdom is threatened by a force that is related to the same kind of threats the Avengers face. The Avengers hear about her and her powers and travel to her world to offer their help. Along the way, they learn about her backstory, and she learns about their heroics. Together, they combine their powers to save her kingdom and in the process, Elsa decides to join the Avengers on some of their adventures in their own universe.