Well, Levi was born in the Underground. His early life was full of struggle. He faced poverty, violence, and a general lack of order. But these difficult experiences also honed his combat skills and his will to survive. As he grew up, he got involved in various battles and gradually became the strong and respected Captain Levi we know. He joined the Survey Corps later, bringing his unique combat abilities to fight against the Titans.
He could also meet Odin. Odin might view Captain America as an interesting mortal from Midgard and might have some words of wisdom or a task for him to complete during his stay in Asgard.
Trust is also a big theme. Given Levi's background and his position in the Survey Corps, it takes time for him to trust someone. So, many fanfictions explore how the reader earns Levi's trust and how that trust evolves into a deeper connection, whether it's a platonic or a romantic one, but always in a non - sexual way.
One idea could be that the reader is a new recruit in Levi's squad. Levi's wife, being kind - hearted, takes the reader under her wing. She helps the reader with training and they form a deep friendship. As time passes, the reader discovers some hidden secrets about Levi's past through his wife, which adds an element of mystery to the story.
One possible plot could be that the kids somehow get transported back in time. Maybe through some magical or mysterious power. They meet Eren and Levi before they've been hardened by war. Eren might be more idealistic and naive, while Levi could be less jaded. The kids could learn about their pasts, their hopes, and dreams, and perhaps even change the course of future events by influencing them in some way.
In a 'Captain Marvel Meet the Avengers' fanfic, there could be an epic showdown against a powerful villain. Captain Marvel's immense power combined with the Avengers' diverse skills would make for an amazing battle. For example, she could team up with Thor to take on an enemy that has the ability to control electricity. Their combined strength of her photon blasts and his lightning strikes would be a sight to behold.