There are numerous storylines that could come out of Loki joining the Avengers in fanfiction. For instance, Loki could have a redemption arc where he gradually becomes a true hero in the eyes of the Avengers. Or there could be a storyline where Loki's past catches up with him and the Avengers have to decide whether to stand by him or not. Also, a fun storyline could be Loki teaching some of the Avengers his magic tricks, which could lead to some comical and also useful situations in battles. He could also introduce the Avengers to some Asgardian customs and technology that they've never seen before, opening up new possibilities for the team.
Well, Loki's a complex character. His mischievous nature would surely shake things up. He might initially cause some distrust among the Avengers. But his intelligence could also be an asset. For example, in battle, he could come up with sneaky and effective strategies that the more straightforward Avengers like Captain America might not think of.
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.
In this fanfiction, Loki might interact most with Hulk. It would be an interesting dynamic as Loki is a master of manipulation and Hulk is, well, the Hulk. Loki could either try to use Hulk for his own purposes, which would be very dangerous, or he could end up on the wrong side of Hulk's anger. But at the same time, it could also lead to some unexpected moments of understanding or at least a begrudging respect between them.
He would probably interact a lot with Thor. Since they are brothers, there is a pre - existing relationship full of complexity. Also, he might interact with Iron Man quite often. Tony Stark's sarcastic nature and Loki's trickery could lead to some really interesting exchanges, either in the form of verbal sparring or working together on some high - tech and magic - combined projects.
In Avengers fanfiction, Loki and Sigyn can be depicted in various ways. One could have Sigyn as a force for Loki's redemption within the Avengers' universe. She might use her connection to Loki to try and sway him from his more villainous tendencies, adding an interesting dynamic to the Avengers' storylines.
Loki could also provide the Avengers with valuable information about otherworldly threats. Since he has connections to different realms, he may know about an upcoming invasion. In return for his help, he could demand that the Avengers clear his name for some of his past misdeeds, leading to an interesting dynamic between them in the fanfiction.
One idea could be that Loki gets turned into a vampire while in the midst of his schemes against the Avengers. This new state gives him different powers and motives. He might struggle with his newfound thirst and also use his vampire abilities to outwit the Avengers in different ways.
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'.