Well, it might show her becoming more independent. With the symbiote, she doesn't have to rely as much on her traditional gadgets. She can trust in her new symbiotic abilities.
The fanfiction might explore her character development through her relationships. In the past, she was mostly alone, but with the Avengers, she has formed deep bonds. These relationships have made her more open and trusting, which is a far cry from her solitary and distrustful past.
It can show her vulnerability. Gaining weight might make her feel less confident in her physical abilities at first. For example, she might struggle with simple acrobatic moves she used to do easily. This can lead to her having to find new ways to be strong, not just physically but also mentally.
One interesting concept could be how the symbiote enhances Black Widow's combat skills. It might give her super strength and agility, allowing her to take on more powerful foes. For example, she could use the symbiote's tendrils to grapple enemies from a distance or quickly scale buildings during a chase.
Fem Harry might develop a more stealthy and calculating side, influenced by Black Widow. This could change the way she approaches problems, instead of just using brute magic force. For example, she could use her magic in more subtle ways to gather information, like a magically enhanced eavesdropping spell.
Writers can make Black Widow believable by staying true to her established character traits. She is known for her stealth and combat skills, so showing her using these effectively during the kidnapping process is important. For example, she could use her martial arts skills to subdue guards or use her spy training to infiltrate a secure location to carry out the kidnapping.
In the comics, Black Widow's death can vary depending on the storyline. Sometimes it's due to a final heroic sacrifice to save others.
It shows a more domestic side of them. Usually, we see them as badass Avengers, but in the fanfiction with a baby, we can see their nurturing and caring sides. For example, Hawkeye might be the one to wake up in the middle of the night to soothe the baby, using his calm demeanor.
In fanfiction, the idea of Spiderman and Black Widow being in love can be really interesting. Some fanfictions might explore how their different personalities would interact in a romantic relationship. For example, Spiderman's youth and idealism contrasted with Black Widow's more complex and sometimes jaded past.
Well, in some fanfictions, their relationship might take on a more romantic angle. Given their shared history and the fact that they are both very attractive individuals. However, in the official Marvel canon, it's more of a platonic friendship. Fanfic writers might play with the idea of what if they had a different kind of relationship, exploring their emotions and how they might interact in a romantic context.
In addition to the Avengers, there could be original characters created by the fanfic writers. These characters could be doctors or nurses taking care of Black Widow in a secret medical facility. Or they could be other agents who have some connection to her past. Sometimes, even villains from the Marvel universe might interact with her while she's in a vulnerable state, either to gloat or to try and make a deal with her.