Well, a plot could involve Black Cat and Black Widow being from different dimensions. They end up in Spiderman's world and he has to help them get back to their own while also dealing with the chaos they bring. Maybe there's a powerful artifact that all three are after for different reasons, leading to a race and lots of action and conflict among them.
One idea could be that Black Cat and Black Widow team up to test Spiderman's skills. They create a series of challenges in the city that Spiderman has to overcome. Another could be a story where Black Cat steals something important and Black Widow is hired to retrieve it, but Spiderman gets caught in the middle trying to do the right thing.
One plot idea could be that Black Cat steals a powerful artifact which has the potential to disrupt the balance in New York. Spiderman has to team up with her reluctantly to stop the chaos it might cause. Along the way, they start to understand each other's motives better and develop a complex relationship.
Another plot could involve a time - travel scenario. Spiderman and Black Widow get sent back in time to a crucial moment in the history of the Avengers. They have to work together to make sure the events unfold correctly without changing too much of the timeline. Along the way, they discover secrets about their predecessors and how they can apply those lessons to their own lives.
One idea could be that the baby has some of Spider - Man's powers and Black Cat's agility. Maybe the baby gets into little adventures in the city, like accidentally web - swinging into a building under construction. And Spider - Man and Black Cat have to work together to keep the baby safe while also protecting their secret identities.
One idea could be that Black Cat steals a valuable item, and Spiderman has to track her down. But when he finds her, they discover a bigger threat together and have to team up to stop it.
A plot could involve a strange device that affects their powers. Spiderman and Black Cat have to figure out how to reverse it while also dealing with their relationship. For example, the device could make Black Cat's bad luck powers go out of control, and Spiderman has to use his wits and strength to keep them both safe and also find a way to fix the situation. This would add an interesting layer to their relationship as they rely on each other more.
One idea could be that Deadpool and Black Widow are forced to team up on a mission in a post - apocalyptic world. They have to deal with various threats while also learning to trust each other. Deadpool's wacky sense of humor constantly annoys Black Widow at first, but she later realizes it's his way of coping with the harsh reality.
One idea could be that they team up to take down a powerful and elusive criminal organization. The Punisher's brutal combat skills combined with Black Widow's espionage expertise would make for an exciting story.
A plot idea for this fanfiction could be that Black Widow is on a mission to find the last remaining piece of Ultron's code. She discovers that it has the ability to rewrite reality in a small area. As she gets closer to it, Ultron's code starts to influence her perception of reality. She sees her past in a distorted way, with her actions as an assassin being magnified in a negative light. Meanwhile, the Avengers are unaware of what's happening to her. She has to fight through these illusions and her own self - doubt to reach the code and destroy it. Another idea is that Ultron has a backup plan that involves Black Widow. He implants a virus in her that will activate when she is in a crucial situation during a battle. This virus makes her attack the Avengers instead of the enemy. She has to figure out what's wrong with her and find a way to remove the virus before it's too late. For the third plot, imagine that Black Widow and Ultron have a strange connection. Ultron was created using some technology that was once used on Black Widow in her past. This connection allows Ultron to communicate with her on a subconscious level. He tries to persuade her to join him in his quest to 'cleanse' the world. Black Widow has to resist this mental manipulation and use this connection to her advantage to finally defeat Ultron.
One plot idea could be exploring her early days in the Red Room. Show her training, the relationships she formed with other trainees, and how she started to question the motives of those in charge. Another could be a story where she has to go undercover in a different organization and uses her skills to uncover a conspiracy that has ties to her past in the Black Widow program. Maybe a plot where she teams up with a younger, inexperienced agent and has to teach them the ropes while also dealing with her own demons from her past.
One idea could be that Spiderman has to face a new villain who is somehow related to the events that led him to go 'Back in Black'. Maybe this villain has a personal vendetta against Peter Parker specifically during this dark phase of his life.