A new superhero created specifically for the fanfiction could be the main character. This character could have a power that is a combination of different Avenger's powers, like being able to have super strength like Hulk and also fly like Iron Man. Their unique abilities would draw the attention of the Avengers, making them the center of a harem - like story.
Peter Parker (Spider - Man) could also be a great main character for this kind of fanfiction. His youthful charm and heroic nature would appeal to characters in both universes. He could be on a school exchange program in Hogwarts (using some multiverse - like excuse) and during his stay, he gets involved with helping the wizards while also catching the eyes of Ginny Weasley, MJ, and Black Widow, creating an interesting harem situation.
I think Spider - Man would make an interesting main character. He's young and has a different perspective compared to the more experienced Avengers. His journey of growing up and becoming a more integral part of the team could be a central theme in the fanfiction. He also has his own set of personal problems like school and his relationships which could add depth to the story.
Captain America might be the main character. He represents the heart and soul of the Avengers in many ways. In this fanfiction, he could be the one leading the team through an emotional crisis, using his unwavering moral compass to guide them and protect the heart of the team and the world they are trying to save.
Iron Man could be the main character. He could use his technological genius to enhance the wings. Maybe he creates a special alloy for the wings that can withstand extreme heat and cold, and also has built - in weapons. He would be constantly tinkering with the wings, trying to make them better, all while using them to fight off new threats.
Obviously, the cook would be one of the main characters. They could be a young and ambitious chef who has always dreamed of working in a place as exciting as the Avengers Tower.
Aragorn could be a great choice. He already has a heroic and noble presence. In a harem fanfiction, he could attract various female characters from different regions of Middle - earth. His skills as a warrior and leader would make him an appealing focus for a harem story.
Gray could be the main character in a harem fanfiction. His cool exterior and his ability to use ice magic in such a unique way make him stand out. He has a bit of a rivalry with Natsu which could add an interesting dynamic to the harem. Characters might be attracted to his brooding nature and his underlying loyalty to his friends and the guild.
Wanda Maximoff could also be the main character. After losing Vision, she could be on a rampage. But then she realizes that she needs to control her powers and use them for the greater good. She could be a key player in trying to bring back the vanished ones and fighting any new threats that emerged after Infinity War.
Captain America could be the main character. Given his leadership qualities, he would be the one to step up and set an example when it comes to using the rectal thermometer. He would be the one to calm everyone down and remind them that it's for the greater good. He might also have some internal struggle with the whole idea, being from a more modest time, but he would do it anyway for the sake of the team.
The main character could be Hermione Granger. Since she is very intelligent and has a strong sense of justice, she could easily be re - imagined as a lawyer. She could use her vast knowledge of magic and her quick thinking to bridge the gap between the two worlds and help the Avengers out. For example, if there was a spell that was being misused in the Avengers' world, she could figure out how to stop it legally.