A possible plot could be that the baby has the ability to phase through objects like Black Cat and also has Spider - Man's spider - sense. The baby starts to show these powers at a very young age, and Spider - Man and Black Cat are both amazed and a bit worried. They then start to train the baby in a safe way, all while trying to keep their family life a secret from the outside world.
Well, perhaps the baby is a target of some villains who want to use its unique genetic makeup (a combination of Spider - Man and Black Cat's traits) for their evil plans. So, Spider - Man and Black Cat have to form an alliance with other superheroes to protect the baby. They also have to deal with the challenges of raising a child in a world full of danger.
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 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 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.
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.
A great fanfiction idea could be set in an alternate universe where Spiderman and Black Cat are part of a super - hero team. They have to work together with other heroes to save the world from a powerful alien threat. Their dynamic within the team would be really interesting to read about.
Maybe in the fanfiction, they decide to start a family. But there are complications due to their superhero lives. They have to find a way to keep their future child safe from their enemies while still being superheroes. This could involve building a secret hideout just for their family.
In the fanfiction, their son could be part of a new superhero team. He brings his unique set of skills, which are a blend of Spiderman's and Black Cat's, to the table. The team might initially be skeptical of him because of his parents' complex reputations. But as they go on missions together, he proves himself to be a valuable asset. His relationship with the team members could also be an interesting aspect to explore, as he tries to fit in while still being true to his own identity.
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 interesting aspect could be the dynamic between their secret identities. Spiderman is a hero with a great sense of responsibility, while Black Cat is more of an anti - hero. Their love story might explore how they deal with their different moral compasses. For example, Black Cat might try to persuade Spiderman to be a bit more flexible in his heroics at times.
One interesting plot point could be how they keep the pregnancy a secret from their enemies. Since they are both in the superhero world, having a pregnancy could be seen as a vulnerability. So, they might have to come up with elaborate plans to hide it while still fighting crime.
In a more fantastical plot, the train itself is alive. The black cat is its guardian. Eve is a dreamer who enters the train's world by accident. The cat has to guide Eve through the different realms of the train, which are full of strange creatures and magical landscapes. And Eve has to learn to trust the cat to find her way back home.