The Hulk is also a good choice. Imagine if the Hulk's anger not only made him stronger but also made him grow to an even more colossal size. Or perhaps he shrinks down and has to deal with his rage in a more confined space, which could be really challenging for his character.
Ant - Man would be a likely candidate as his power already involves size - changing. In fanfiction, his ability could be enhanced or malfunction, leading to all kinds of interesting scenarios. For example, he could accidentally make all the other Avengers the same size as him, creating a whole new dynamic for the team.
Thor could be an interesting focus too. If his godly powers were somehow affected and he grew or shrank, it would change his fighting style completely. He might have to rely on his wits more when he's small, or when he's large, he could cause even more destruction with his mighty hammer strikes.
Tony Stark is a likely focus. His intelligence and experience with technology, especially time - travel technology in the movies, make him a central figure. He could be the one to figure out the mechanics of time travel and lead the Avengers in their time - traveling adventures.
Tony Stark is likely to be a focus. His flamboyant and playboy - like personality could lead to some really interesting scenarios when affected by a love potion. He might make grand gestures towards someone he wouldn't normally be interested in. Another likely character is Natasha Romanoff. Her complex past and guarded nature could create a great contrast when the love potion breaks down her walls.
Hulk (Bruce Banner) could be the focus. Since his body is already in a state of flux between his human and Hulk forms, any changes in his diet or a scientific experiment gone wrong could potentially cause him to gain weight in his human form, which would add an interesting twist to his character.
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.
Captain America would be a likely candidate. He's all about justice and if he thought Spider - Man was doing something wrong, he'd go after him. Also, Iron Man. He's often quick to act and if he saw Spider - Man as a threat to his technology or the safety of the world, he'd be on the chase. Black Widow too, as she's used to tracking down targets and if she was given the order to bring in Spider - Man, she'd do it.
Captain America is very likely to be featured. He has that paternal, moral - compass kind of vibe. He would be the one to gently tickle the 'daughter' as a way of showing his soft side. Thor could also be a popular choice. His big - hearted and sometimes child - like nature would make for some great tickle - related antics with the 'daughter' figure.
Definitely Tony Stark. His complex personality, his wealth, and his role as a sort of father figure in some scenarios make him a great focus. He can be shown as mentoring the younger Avengers or having heart - to - heart talks with them about responsibility.
Definitely Torunn. She has a great backstory being Thor's daughter, and her powers are really interesting. You can explore her journey of living up to her father's name and forging her own path as a hero.
Captain America could be a likely character to be betrayed. He represents the old - school values of the Avengers, and in a more modern, cynical world within fanfiction, some might see his ideals as outdated and turn against him. Also, Tony Stark. His wealth and sometimes over - confident attitude could make some of the other Avengers, especially those with more humble backgrounds, feel resentful and potentially betray him. And Thor, being an Asgardian, might be betrayed due to the other Avengers' mistrust of his alien nature or the potential threat Asgard could pose.
I think Steve Rogers would be a likely ally. Steve is all about justice and doing what's right, and Castiel has a similar sense of duty in his own way. They could bond over their strong moral compasses.