Captain America could also be the main character. He comes from a different time and might struggle to adapt to the modern concept of happiness. His journey could be about reconciling his old - fashioned values with the new ways of finding joy and contentment in the present world of the Avengers.
One idea could be that the Avengers discover a new form of happiness that is not based on victory but on self - discovery. They each go on a journey to find what truly makes them happy outside of their superhero personas.
I'm not sure specifically as I haven't read it. But generally, a fanfiction often explores different scenarios or relationships within the world or characters created by the original work. It might be about a character's journey to find a more profound or different kind of happiness.
You could try searching on popular fanfiction websites like Archive of Our Own (AO3) or Fanfiction.net. These platforms usually have a wide range of fanfictions, and there's a good chance you'll find it there.
In some fanfictions, the 7th Avenger could be a time - traveler from the future. This person has come back to prevent a major disaster that the Avengers couldn't handle without their help. They know all about the future events and use this knowledge to guide the team, but also have to be careful not to change the timeline too much.
It could be a super - powered villain who has a vendetta against the Avengers. Maybe he sees Peter as a new and vulnerable target since he's just joining the team. This villain might have some connection to Peter's past that makes him especially interested in causing trouble for Peter and by extension, the Avengers.
Maybe Thanos somehow discovers the wizarding world and decides he wants to add the power of magic to his collection of Infinity Stones. He could be the main villain, and Harry, with his understanding of both magic and his new Avengers - like abilities, would have to lead the fight against Thanos to protect both worlds.
It could be Captain America. He has a strong sense of justice and kindness. So, if he found a dog, he would surely take it under his wing and protect it. Maybe he'd even train the dog to be a mascot for the Avengers.
A young gay man who has just come out and is full of passion to fight for his rights could be one of the main characters.
Another character he could interact with is Thor. Thor's otherworldly nature and his powerful abilities would be quite a contrast to Stiles. Maybe Stiles could help Thor understand some earthly concepts while Thor could teach Stiles about Asgardian culture and power.
Howard the Duck. It would be really unexpected. He could bring some comical relief but also have some hidden knowledge about the disks that could be crucial. For instance, he might know of a weakness in a powerful enemy's disk - based powers.