It could be Captain America. He represents leadership and moral compass. When the Avengers meet Sleipnir, he would be the one to decide whether to trust it or not. His strong sense of right and wrong would guide the team's actions in relation to Sleipnir, and his combat skills would be useful in any battles that occur during the encounter with Sleipnir.
Thor could be the main character. Since he has a connection to Norse mythology and Sleipnir is from that realm. He would be the one most likely to understand Sleipnir and communicate with it.
One possible plot could be that the Avengers are on a mission in a realm where they accidentally stumble upon Sleipnir. They initially mistake it for a threat but then realize it has its own intelligence and might be an ally. Maybe Sleipnir can help them travel through different dimensions to stop a greater evil that is spreading across the multiverse.
One possible plot could be that Sleipnir, the eight - legged horse from Norse mythology, somehow ends up in the Marvel universe. The Avengers initially mistake it for a threat. But then they discover that Sleipnir is being chased by some dark Norse entities. The Avengers decide to protect Sleipnir and in the process, they learn about Norse magic and mythology, and how it can be both a help and a hindrance in their own universe.
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.
The Green Goblin could be a great main villain. He has a long - standing feud with Spiderman, and when the Avengers get involved, it could make the situation even more complex. His flying glider and pumpkin bombs would pose a serious threat to both Spiderman and the Avengers.
It could be Iron Man, Tony Stark's father is Howard Stark. So Tony would have a very emotional and complex relationship in this meeting. He might be the main character as he gets to know his father in a different way and learn more about his past.
Captain America could be the main character. He's always been a leader, and in the aftermath, he could be the one trying to hold the remaining Avengers together. He would be dealing with his own sense of loss and responsibility. He might also be trying to figure out how to prevent future disasters like Infinity War.
Matt could be a new superhero in the fanfic. Maybe he has some unique powers or abilities that haven't been seen before. He might be a street - level hero who has a different perspective compared to the Avengers.
In this fanfic, the main antagonist could be a scientist who has been corrupted by his own experiments. He creates some sort of energy - based monsters that are immune to normal attacks. Captain Marvel and the Avengers have to figure out how to stop these creatures and the mad scientist behind them.
Well, it might also involve a lot of character interactions. The serious and often Earth - bound Avengers could learn a lot from the more laid - back and space - faring Guardians. Maybe Thor and Drax could bond over their love for physical combat and have some sort of friendly competition. And Black Widow could teach Gamora some new spy techniques.
Well, in this fanfic, maybe the Avengers would initially be suspicious of Matt. They might think he's an ordinary guy at first. Then Matt could show off some unexpected skills that catch their attention, like his martial arts abilities or his detective skills if he's Daredevil in this story.