Ash would probably be very curious about the Avengers' powers. He might ask a lot of questions and want to see them in action. In return, he could show them his Pokémon and how they fight.
Fem Harry would likely approach Ash with great enthusiasm. She would probably ask him a million questions about his Pokémon adventures, like how he caught his first Pikachu. Ash, being friendly, would happily answer. And in return, he might be curious about Harry's magic world, asking things like how she can fly on a broomstick.
In a fanfiction where Ash Ketchum meets the Avengers, there would be a huge clash of different worlds. Ash, being from the Pokémon universe, would bring his unique sense of adventure. The Avengers, on the other hand, are used to dealing with more technological and superhuman threats. Ash could introduce them to the concept of Pokémon, and perhaps some of the Avengers would be interested in having a Pokémon partner. For example, Tony Stark might see the potential in using Pokémon for his inventions. It would be a story full of surprises and new friendships.
Ash and Serena may also interact in a way that shows their growth as characters. Serena might be more assertive in her feelings for Ash, and Ash might be more perceptive to Serena's emotions. They could have a picnic together, where Serena feeds Ash some of her homemade treats, and they exchange sweet nothings, their relationship growing deeper with every interaction.
Snape would probably start off being rather aloof. He'd observe the Avengers' behavior and abilities before making any real moves to interact. He could be sarcastic towards them, especially if they make assumptions about him or his magic. But as time goes on, he might start to see their value and open up a bit more, sharing some of his magical knowledge.
Naruto would interact in a very enthusiastic way. He'd be in awe of the Avengers' powers and would want to learn from them. At the same time, he'd show off his own ninja abilities. He might bond with Hulk over their shared 'rage' - like Naruto has his nine - tailed fox anger and Hulk has his own. He'd also try to cheer up the more serious Avengers like Black Widow with his positive attitude. And he'd probably try to get them all to do some ninja training together.
They might start by comparing powers. Superman's super strength and flight would be quite a topic. Thor would likely want to have a friendly competition with Superman in terms of strength. Then they could move on to sharing their experiences. Captain America could talk about his battles during World War II, and Superman could share his adventures in saving Metropolis. They could also team up to fight a new, powerful villain that neither could defeat alone. For example, a villain with the ability to nullify Superman's powers and hack into the Avengers' technology.
Supergirl would probably start by introducing herself and her powers. The Avengers would likely be in awe of her abilities. They might ask her to join them on a mission right away. Supergirl could say yes and then start showing off her skills during the mission.
In fanfiction, Venom could interact with the Avengers in many ways. For example, he might initially be seen as a threat and the Avengers try to subdue him. But then they could discover that he has some common enemies, like certain powerful villains who are a menace to both him and the Avengers. So, they could form an uneasy alliance.
The Avengers and the Eternals could interact in a variety of ways. The Hulk might be intrigued by Gilgamesh's strength. Black Widow could try to figure out the Eternals' motives. And on the Eternals' side, Sersi might befriend Scarlet Witch and learn about her magic. There could also be some friendly competitions between the two groups, like a race between Quicksilver and Makkari.
They might be in awe of each other at first. The Fanon Avengers could look up to the Canon Avengers as the originals, while the Canon Avengers might be intrigued by the unique takes on their characters in the Fanon world. They could start by comparing their powers and sharing stories about their adventures.