One idea could be that Naruto is somehow transported to the Marvel universe and meets Natasha Romanoff. They could team up to fight a common enemy that has powers similar to those Naruto has faced in his own world, like a villain with the ability to control shadow clones.
Another plot could be that there is a threat that endangers both the ninja world and the Marvel world. Naruto and Natasha are chosen as representatives of their respective worlds to find a solution. Along the way, they discover that their worlds have some hidden connections in the past, and they have to work together to uncover the mystery and stop the threat.
One idea could be a story where they discover their sibling bond in a rather unexpected way. Maybe they both get kidnapped by a common enemy who reveals the truth about their relation. They then have to work together to escape and along the way, they share memories from their past that they didn't know were connected. For example, they might find out that they both had a favorite hiding spot as children in two different locations that were actually part of the same secret family place.
One idea could be that Naruto finds himself in Natasha's world through some sort of magical portal. He has to adapt to a completely different environment and learn about her world's way of fighting and espionage. Meanwhile, Natasha is intrigued by Naruto's unique abilities like his shadow clones and Rasengan. They could team up to fight a common enemy that threatens both their worlds.
A plot could revolve around a kid Natasha joining the Avengers on a time - traveling adventure. They go back in time to stop an event that could change the future of the Avengers. Along the way, Natasha has to use her wits and natural skills to overcome various obstacles, and she forms a special bond with the other Avengers members during this journey.
In a fanfiction plot, Natasha could become a healer on the war front where Andrei is fighting. She disguises herself as a boy to be there. Andrei gets injured and she takes care of him. This brings them closer in a different way than in the original story. They then have to deal with the secrecy of her identity and the challenges of war while falling deeper in love.
One plot idea could be that Natasha and Steve are on the run from some new threat while Natasha is pregnant. They have to constantly find safe places to hide and also deal with the physical and emotional challenges of the pregnancy. Steve is overly protective and Natasha is trying to be as tough as ever, which leads to some cute and tense moments between them.
One idea could be that Natasha is on a mission in the wizarding world. Maybe she's tracking down a dark wizard who has some sort of connection to a powerful and dangerous Muggle weapon. Harry Potter could help her navigate the magical aspects of the case, like dealing with spells and magical creatures.
Another idea is that Peter gets lost in a dangerous part of the city and Natasha has to find him. Along the way, she discovers his unique abilities and how they can be used in different ways. She then starts training him to be more in control of his powers while also protecting him from the threats in that area.
One idea could be a story where Tony and Natasha have to go undercover as a couple on a mission. They start to develop real feelings during the process. For example, they might have to attend a high - society event and deal with all the pretenses while also trying to uncover a hidden enemy. Another plot could be set in a post - apocalyptic world where they are two of the few survivors and have to rely on each other completely to stay alive and rebuild society. Or perhaps a story where they both get trapped in a virtual reality world created by an enemy and have to find a way out together while also facing their own inner demons.
One idea could be Natasha going back in time to prevent a major event that led to the loss of many lives. For example, she could stop a certain villain from getting a powerful weapon in the past. This would create a whole new timeline and she would have to deal with the consequences of changing history.
A plot idea could be that Naruto discovers a new, ancient form of chakra that has been lost for centuries. He then has to train to master this chakra while also dealing with the political implications within the Hidden Leaf Village. Some elders might be against him exploring this new power for fear of the unknown, but Naruto, being Naruto, is determined to master it for the good of the village.