One possible plotline could be that the boy, after turning into Black Widow, has to adapt to her skills and abilities. He might find himself in the middle of a spy mission and has to use her combat and espionage training to get out of sticky situations. Another could be that he has to deal with the memories and emotions that come with becoming Black Widow, like her past traumas and relationships.
A plotline could involve the boy - as Black Widow - being transported to a different dimension where he has to prove himself as the real Black Widow. He'd need to learn quickly how to use her weapons and gadgets, like the widow's bite. Along the way, he might discover new aspects of Black Widow's character that were previously unknown, and this could lead to a change in how the other characters view her.
One possible plotline could be that Izuku is somehow transported to Black Widow's universe. There, he has to adapt to a world without quirks and learn to use more traditional combat methods.
One possible plotline could be that Naruto is somehow transported to the Marvel universe and he meets Black Widow. They then team up to fight a common enemy, perhaps an evil ninja - like villain who has some sort of connection to Naruto's world but is also causing trouble in the Marvel world.
In a scientific experiment gone wrong scenario, a boy volunteers for a cutting - edge experiment. There's a malfunction in the technology that was supposed to enhance human abilities. Instead, it alters his genetic makeup and he turns into a girl. He struggles to come to terms with this new physical form while also dealing with the scientific community trying to fix their mistake.
One possible plotline could be that Black Widow somehow gets transported to the Naruto universe. She could use her combat skills and spy training to help the ninja in Naruto's world with a particular mission, like dealing with a new and dangerous enemy that has techniques similar to those in the Marvel world. For example, an enemy that can control minds like some villains in the Marvel universe.
His personality might become more reserved and calculating, like Black Widow. He'd be more cautious in his interactions and less likely to trust easily.
A plotline could involve Percy Jackson being sent on a quest that somehow involves the modern world and S.H.I.E.L.D., where Black Widow is involved. They meet, and Black Widow is initially skeptical of Percy's powers and claims. But as they face various challenges together, she realizes his abilities are real. The fanfiction could explore how their two very different worlds and personalities interact and eventually lead to a romantic connection. There could be elements of Percy teaching Black Widow about the demigod world and Black Widow showing Percy some of her spy skills.
Perhaps a story where Black Widow accidentally stumbles into the wizarding world and is mistaken for a threat at first. But then she proves herself useful in helping to solve a mystery within Hogwarts, like a series of strange disappearances that are actually related to a dark plot. This would be a story full of mystery, adventure, and no 'lemon' content.
Another plotline could involve a betrayal within the group that causes Elsword to turn dark. For example, if he discovers that one of his closest allies has been secretly working against him all along, the shock and anger could drive him to the dark side. He might then start to seek revenge on not only the betrayer but also those he perceives as being part of the conspiracy.
Perhaps in the fanfiction, Deku turns into an adult due to a quirk malfunction. As an adult, he could struggle with his changed appearance and how his friends and family react. He might also find that his relationships with other characters, like Ochaco, change in unexpected ways as he is now an adult in a different stage of life.
Losing someone she loves deeply could also lead her to turn evil. If, say, Hawkeye was killed and she blamed the Avengers or the world in general for not being able to protect him, she could become vengeful and start to act in an evil way. Grief and anger can often twist a person's moral compass, and Black Widow would be no exception.