One possibility is that there would be a lot of mistrust at first. Yelena has her own agenda and the Avengers are a tight - knit group. Maybe she tries to infiltrate them for some reason related to her past in the Red Room. For example, if she thought the Avengers had some information about the remaining Black Widow - related operations.
In fanfiction, when the Avengers meet Yelena, there could be a lot of action. Maybe Yelena tries to prove herself to them. She might show off her combat skills and they would be impressed.
When Yelena meets the Avengers in fanfiction, her interaction depends on the writer's take. She could be standoffish, having been trained in a different way than the Avengers. However, she may also be drawn to their sense of justice. She might start by observing them from a distance. As she gets closer, she could offer her unique skills to help with a mission. Her interactions could range from tense exchanges with the more serious Avengers like Thor, to more light - hearted banter with the likes of Star - Lord if he's part of the group. It's all about how the fanfiction writer wants to develop their relationship.
Well, they might initially be cautious of her. Since Yelena has her own set of skills and a bit of a wild card nature. Maybe she challenges them to a friendly sparring session to show off her combat abilities.
Steve Rogers might also be an ally. He has a sense of honor and justice that Yelena could come to respect. He could see her potential and try to guide her in the right direction, especially if he saw her as a way to carry on Natasha's legacy in some way.
Thor could also be in the mix. His god - like powers and different cultural background compared to Yelena would create an interesting dynamic. Maybe Yelena is initially in awe of his abilities or she challenges him in some way, leading to an engaging interaction.
Well, Steve Rogers might be a bit confused at first. He's from a different era and fanfiction is a very modern concept. But he'd probably be polite about it and say something like 'It's interesting to see how fans imagine different scenarios with Yelena.' Tony Stark, on the other hand, would likely be more intrigued. He might chuckle and say 'This is some creative stuff. I wonder what else they come up with about Yelena.' And Thor would probably just be like 'This is a strange Midgardian thing, but if it makes people happy, then so be it.'
Tony could use his technology to analyze the popularity trends in Yelena fanfiction. He might say 'I can see which scenarios are the most popular in these fanfics about Yelena.' Natasha might take some of the positive portrayals in the fanfiction as a compliment to her sister. She could say 'This shows that Yelena is making an impact.' Thor might not fully understand but he could share the fanfiction stories in Asgard as a strange but interesting Midgardian thing.
Yelena could bring a fresh perspective to the Avengers. Her skills as a Black Widow would be something they notice right away. She might start off being a bit stand - offish, but as she sees the Avengers' good intentions, she warms up. They could exchange combat techniques and strategies. And she might even teach them some of the more underhanded tactics she learned during her training.
One possible plotline could be Yelena initially being distrustful of the Avengers. Maybe she has been taught to view them as enemies, but as she gets to know them, she realizes they have similar goals. For example, she might find common ground with Natasha's former teammates in their fight against evil.