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.
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.
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.
A common theme is the power dynamic. Angelique often has a certain power over Barnabas, whether it be through magic or her strong will. Another theme is the idea of forbidden love. Their relationship is complex and often goes against what society or their families might expect. Also, revenge is a theme. Angelique's actions are sometimes driven by a desire for revenge against Barnabas for perceived wrongs.
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.
Barnabas Collins' love story is quite complex. He had a deep and passionate love for Josette. Their love was thwarted by various dark forces and family secrets in the Collins family. Their relationship was filled with both tender moments and great tragedies, which made it a very captivating and somewhat melancholy love story.
Their love was forbidden mainly because of Barnabas' condition as a vampire. In fanfiction, we could see them trying to overcome this obstacle. For instance, Barnabas might search for a cure so that he can be with Josette without the threat of harming her.