Perhaps Harry discovers a way to travel to the Marvel universe and becomes enamored with the Avengers. He might then try to prove himself to them, showing off his magic skills and offering to help them in their battles.
If Harry Potter became an Avenger, it could lead to interesting crossovers and storylines. His experiences in the wizarding world could give him a different perspective in battles against villains.
One idea could be a crossover where the Avengers end up in the wizarding world of Harry Potter. They might be confused by the magic at first but then start to learn and adapt. For example, Iron Man could try to figure out how to incorporate magic into his tech. Another idea is that Harry Potter and his friends are brought to the Avengers' universe and they help the Avengers fight a new kind of threat that has both magical and non - magical aspects. Maybe Voldemort somehow teams up with a supervillain from the Avengers world.
It could potentially be a great fanfic. It might bring together two very different fandoms in a creative and engaging manner. Imagine Harry getting involved with the Avengers to fight a common enemy that threatens both the wizarding world and the normal world. The fanfic could also delve into the relationships Harry forms with the Avengers, like a mentorship with Tony Stark or a friendship with Captain America, which would make it stand out.
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.