One great 'Hulk x Loki fanfiction' is 'The Unlikely Bond'. It shows an interesting dynamic between the two characters where they are forced to work together on a mission. Their clashing personalities create a lot of tension and some really humorous moments.
It could be someone who is manipulating Loki to kidnap the Hulk. Maybe an even more powerful villain from the Marvel universe, like Thanos. He might be using Loki as a pawn to get the Hulk for his own evil purposes.
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.
Well, perhaps Loki kidnaps the Hulk to use his strength in some scheme against the Avengers. Maybe he wants to use the Hulk to break into a secure location or as a weapon against Thor.
A common theme is She - Hulk's struggle to balance her normal life as Jennifer Walters and her superhero identity. For example, in many fanfictions, she has to deal with legal cases during the day and then rush off to save the world at night.
One great 'Avengers She - Hulk Fanfiction' is 'She - Hulk's Adventure in the Avengers Tower'. It has an interesting plot where She - Hulk helps Iron Man upgrade the security system while dealing with her own insecurities within the team.
Future Loki might interact with the Avengers in a cautious way. He knows their past opinions of him and tries to prove himself. He could start by sharing small bits of useful information. For example, he might tell Iron Man about a weakness in a new enemy's technology.
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.