Well, Thor might be a bit shocked at first. He'd probably be amused by some of the wilder interpretations of his brother in the fanfiction. Captain America, on the other hand, would likely be confused by the whole concept as it's so different from the traditional stories he's used to. Iron Man would probably make sarcastic comments about how creative the fans are in coming up with these stories.
If Loki impersonated Thor, the real Thor would be furious when he found out. The other Avengers, like Iron Man, might initially be confused but then quickly realize something is off. Iron Man would likely use his technology to detect the impostor. Hulk might just smash first and ask questions later when he senses something wrong with 'Thor'.
The OC might be terrified at first. After all, being kidnapped by Loki is no joke. But they could also quickly gather their courage. Maybe they would try to talk to Loki, to understand his motives. If the OC is smart, they would know that Loki has his own reasons for doing things and might be able to find a way to reason with him.
Tony Stark would probably be very intrigued. He'd want to study Loki's Jotun form, maybe even make some snarky comments about his blue skin. Steve Rogers, on the other hand, would be more cautious. He'd be ready for any potential threat that Loki might pose.
When the Avengers meet Lady Loki in fanfiction, their reactions would vary. Iron Man would probably start analyzing her, trying to figure out her weaknesses. Captain America, being the moral compass, would try to give her a chance to prove herself, but also be ready to fight if she betrays them. And Thor, he would have a mix of emotions, part anger from past experiences with Loki and part curiosity about this new form of his 'brother'. The more impulsive members like Hulk might just see her as a threat and react accordingly.
In this fanfiction scenario, their reactions could vary greatly. Natasha Romanoff would likely be sizing up the situation immediately, trying to figure out how this new knowledge can be used to their advantage. Clint Barton might be a bit more hesitant as he's had his own experiences with Loki's mind - games. And Bruce Banner would be worried about the potential danger Loki's true form poses, especially if it has some kind of power that could set off the Hulk.
Some of the Avengers, like Thor, might have a more complex reaction. Since he is Loki's brother, he may feel a sense of responsibility towards the children. Others like Captain America would be cautious but fair, trying to assess if they are a threat. Iron Man, on the other hand, would probably be sarcastic and make some snarky remarks while sizing them up.
Thor might be conflicted. He'd see his nephew/niece but also remember Loki's treachery. Iron Man would probably be sarcastic at first, making remarks about the family genes. Captain America would be more cautious and diplomatic, wanting to give her a chance but also being on guard. Black Widow would try to read her body language and motives right away.
Thor would be a likely candidate. Since he is Loki's brother, he already has a connection with him. Thor might be the one to convince the other Avengers to give Loki a chance and be the first to reach out and try to befriend him in this new situation.
Loki could also show signs of fear, though he would try to hide it. Inside, he knows the power that Thanos wields. But on the outside, he would maintain a facade of bravado. His reactions would be a mix of his natural cunning and the instinct to survive in a very dangerous situation.