The fanfiction could start with Sleipnir appearing in the middle of New York City, causing chaos. The Avengers assemble to deal with it. Iron Man tries to scan it, Captain America is ready for a fight if needed. But then they find that Sleipnir is intelligent and is trying to warn them of an impending doom from an ancient Norse evil. So, they team up with Sleipnir to stop this evil from destroying both the human world and the Norse realms.
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.
Maybe in the fanfiction, Loki brings Sleipnir to the Avengers as a sort of peace offering. The Avengers are skeptical but Thor, being familiar with Sleipnir, convinces them to give it a chance. Sleipnir then has some special powers that help the Avengers in a battle against a new, powerful enemy. For example, its ability to travel between the nine realms allows the Avengers to quickly reach different locations and surprise their foes.
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.
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.
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.
Well, the plot could involve the Avengers initially being on a mission when they accidentally stumble upon Fenrir. Maybe they think it's a new threat created by an enemy like Loki. They would then try to fight it, but soon realize that Fenrir has its own motives and might not be entirely evil. It could lead to an alliance of sorts where the Avengers and Fenrir team up to face a greater evil that's endangering the whole universe.
One possibility is that the Avengers are facing a threat related to some old Hydra experiments that Howard Stark was once involved in. They meet him to gather information about the experiments, like the location of hidden files or the identities of key Hydra operatives from that time. Howard Stark, being the brilliant mind he is, joins forces with the Avengers to stop Hydra's resurgence.
Well, perhaps the Avengers are summoned to the Hamilton - era by some magical or time - rift related incident. Black Widow could blend in easily and start gathering intelligence on the political situation. Thor might be mistaken for a god, which could either cause chaos or inspire the people. And Hamilton, being the ambitious man he is, might see an opportunity to use the Avengers to strengthen his own position, leading to some interesting power dynamics and moral dilemmas within the fanfiction.
Perhaps Skip Westcott is in trouble and the Avengers come to his rescue. He might be in possession of some important information that the bad guys want. The Avengers meet him to protect him and get that information before it falls into the wrong hands. For example, he could have knowledge about a new weapon being developed by a supervillain. And the Avengers need to keep that knowledge safe and use it to stop the villain.
Perhaps in the fanfiction, the Avengers meet future Morgan Stark who has been trying to carry on her father's legacy. She might be leading a new group of heroes, and the Avengers could team up with them to face a common enemy. For example, she could have developed advanced Iron Man - like suits for her team. The story could also explore how the Avengers react to seeing the grown - up daughter of Tony Stark and how she has been influenced by her father's sacrifice.
In this fanfiction, perhaps the rogue Avengers are hiding out when they hear baby Peter crying. They initially think it's a trap but then realize it's just an innocent baby. Some of the Avengers might want to take him with them because they see his potential, while others are more hesitant due to their own precarious situation.
One possible plot could be that the Avengers are facing an extremely powerful new enemy that they can't seem to defeat. Captain Marvel arrives on Earth just in time. At first, there are some misunderstandings and clashes of egos among the Avengers and Captain Marvel. But as they start to work together, they realize each other's strengths. Captain Marvel's raw power combined with the Avengers' teamwork strategies slowly turn the tide of the battle. Eventually, they defeat the enemy and Captain Marvel decides to stay on Earth for a while to help the Avengers train for future threats.