It could be Captain America. He is often the one to reach out and try to make peace or form alliances. He would see the potential in the Maximoff twins and approach them with an open mind and a desire to work together for the greater good.
It's also possible that Wanda herself would initiate contact. If she and Pietro were on a mission to find a group that could help them control their powers better or use them for good, she might sense the Avengers' presence and reach out to them. She could be the one to break the ice and start the conversation about potential cooperation.
I think it might be Nico. Given his experiences with different types of beings in his own world, he might sense the supernatural nature of the Winchesters and approach them first, either out of curiosity or because he thinks they could be allies in his ongoing battles against the forces of the underworld.
Well, it could start with a misunderstanding. The Avengers could mistake the Maximoff twins' actions for something malicious. However, as the story progresses, they would come to realize that the twins have good intentions. They might team up to fight a common enemy, like a new super - villain. During the battles, the twins' powers, Pietro's super - speed and Wanda's magic, could be crucial in turning the tide in the Avengers' favor.
Iron Man and Batman might be the most likely to clash. Iron Man is all about high - tech and showing off his latest inventions. Batman, while also using technology, is more of a brooding, solitary figure. Iron Man might see Batman as a bit of a relic or too old - fashioned in his methods. Batman, in turn, could view Iron Man as a bit too flashy and irresponsible with his power. Their egos could get in the way of them working together initially.
It could be Louis. He has always seemed very connected to the group's roots. In fanfiction, he might realize how much the fans miss the band and how much they all still have in common musically. So, he starts reaching out to the other members one by one to see if a reunion is possible.
Goku would be more likely. He is always open to new friendships and challenges. He has a curious nature and would be intrigued by Kefla's power. So he might approach her first to test her strength or just out of friendly curiosity.
Annihilus could be a likely villain. He is a powerful being from the Negative Zone. In the fanfiction, Annihilus could be drawn to Earth because of the energy signature created when the Avengers and Captain Marvel meet. He sees Earth as a prime target for conquest. His powerful cosmic control rod gives him an edge in battle. The Avengers and Captain Marvel have to quickly learn about his weaknesses and use their combined powers to stop him from destroying Earth and spreading his reign of terror across the universe.
It could be Percy. Maybe he's bored one day and just randomly starts messaging people in an online group related to Greek mythology. He comes across Annabeth's profile and is immediately drawn to her intelligent - sounding posts. So he decides to send her a private message to start a conversation.
It could be Naruto. He's known for his boldness and his ability to reach out to people. He might be the one to first notice Korra's unique qualities and be drawn to her, starting conversations and gradually building a connection that could turn into a romantic one.
Doctor Strange and Ajak. Doctor Strange is well - versed in the mystic arts and Ajak has a lot of knowledge about the Eternals' history and their connection to the Celestials. Their knowledge areas could intersect, and they would likely be the first to start sharing information. Maybe Strange senses some strange energy related to the Eternals and approaches Ajak to find out more.
Thor would be the most likely. Since Thor has a connection to Norse mythology already, he would be familiar with Sleipnir. He might see Sleipnir as a symbol of his home realm and would quickly bond with it. They could communicate in a way that the other Avengers can't, sharing stories of Asgard and the great battles there.