Well, it could be that Black Widow is on a solo mission when Thanos captures her. Thanos, seeing her as a formidable opponent despite not having superhuman powers like some of the others, decides to keep her alive. He may want to break her spirit and make her join his side. Black Widow, on the other hand, is constantly looking for ways to escape and send a signal to her friends. She endures a lot of mental and physical torture from Thanos and his minions but remains steadfast in her loyalty to the Avengers.
In a fanfiction, perhaps Thanos captures Black Widow because he discovers she has some knowledge about the Infinity Stones that he doesn't. He tries to extract that information from her through various means, but she resists strongly. She might use her spy skills to try and outwit Thanos even while in captivity.
One possibility could be that Thanos captures Black Widow to use her as bait for the other Avengers. He might think that her comrades will come to her rescue and he can then ambush them all at once.
The Hulk, if he was in his Bruce Banner form, would be very worried. He'd use his scientific knowledge to figure out a way to counter Thanos' power. But if he was in his Hulk form, he'd just be enraged and want to smash his way to Black Widow. The other Avengers like Thor would also be determined to rescue her. Thor, with his newfound strength and his axe Stormbreaker, would be ready to face Thanos head - on to free Black Widow.
The plot could start with Black Widow being sent on a mission to understand Thanos' motives. During their encounters, Thanos starts to develop an unexpected interest in her. Black Widow, initially seeing him as a threat, gradually realizes there's more to him.
One possible plot could be that Kairi is in a strange and dangerous realm where the black tentacles belong to some sort of dark entity. She has to use her courage and any latent powers she has to try and break free. Maybe she discovers that the tentacles are being controlled by an old enemy who she must then defeat in order to escape.
One possible plot could be that in an alternate universe, Loki and Black Widow have a mission together. During this time, they develop unexpected feelings for each other. Then, Black Widow discovers she is pregnant. This leads to a whole series of events where they have to deal with both their relationship and the challenges of having a baby in a world full of danger and intrigue.
One possible plot could be that Thanos sees Loki as a tool that has failed him. So, he subjects Loki to various forms of physical and mental abuse to break his spirit. Loki, being a survivor, tries to find ways to outwit Thanos even in his weakened state.
One possible plot could be that Harry Potter, as the son of Black Widow, discovers his mother's secret past in the wizarding world. He might inherit some of her unique skills, like her combat abilities, and use them to face new threats in Hogwarts. Maybe he forms a special group of friends who are intrigued by his strange heritage and they all embark on adventures together.
One possibility is that the story focuses on Black Widow's internal struggle as she tries to get back to her normal self after the concussion. She might have to face her own fears and insecurities, and the other Avengers could play different roles in helping her through it. For example, Captain America might offer her words of encouragement, while Tony Stark could use his technology to monitor her recovery. And Hawkeye could be there as her old - time friend to cheer her up. The story could also involve some villains trying to take advantage of her weakened state, which would add some action and suspense to the fanfiction.
One possible plot could be that Black Widow, seeing Peter's potential and also his need for a more stable family situation after the death of his uncle, decides to take him under her wing. She could train him in more stealthy and psychological aspects of being a hero, different from what he learns from Tony Stark. For example, she might teach him how to extract information from enemies without using brute force.
One possible plot could be Percy finding out he has a connection to Black Widow. Maybe he discovers some hidden powers or abilities that are related to her skills. He could then face challenges in the demigod world where his newfound 'Black Widow - like' traits help him out, like enhanced combat skills or strategic thinking.
In a 'Captasha captain meets black widow' fanfiction, the plot might involve Captasha being a new hero in the Marvel universe. Black Widow is sent to assess if Captasha can be trusted. They meet in a secret S.H.I.E.L.D. location. At first, Black Widow is skeptical, but as they train together, they develop a mutual respect and end up working together to stop a terrorist organization that has stolen dangerous technology.