Perhaps there's a treasure hunt involved. The pirates believe America has some clue to a great treasure because of his unique position in the world. So they take him along on their adventure, and along the way, there are lots of comical misunderstandings as America tries to explain modern concepts to the pirates.
America has a lot of cultural influence in the real world, and this is often translated into the Hetalia universe. His pop culture, such as movies, music, and fast food, are things that could potentially draw other characters towards him. In fanfiction, this can be shown as the other characters having a fascination or love for America because of these aspects.
Some 'Everyone Loves America' fanfictions could focus on the cultural exchange aspect. America being a melting pot of cultures, fanfictions might show how other nations in the Hetalia universe are intrigued by his diverse cultural elements. It could also be about how America's pop culture spreads and influences the other characters, creating a situation where they all seem to have a certain fondness for him because of it.
One great fanfiction could be 'The Pirate's Treasure'. In it, Pirate England and America go on an adventure to find a hidden treasure. It has a lot of action scenes and also some interesting character interactions. For example, their banter while on the ship is really engaging.
A political intrigue plotline could also be fascinating. Maybe there's a situation where they have to work together to solve an international crisis, but there are other countries trying to undermine their efforts. It would involve a lot of strategy and diplomatic maneuvering between the two characters, with some tense moments and perhaps a budding respect for each other's abilities.
Adventure is a theme that shows up as well. They could go on adventures together, exploring different parts of the world or even fictional realms. And of course, there's the theme of self - discovery. In these fanfictions, Fem America and Fem England often discover new aspects of themselves through their interactions with each other, whether it's their hidden talents or their true feelings.
I'm not sure of specific fanfics with this exact combination, but you can try searching on popular fanfic platforms like Archive of Our Own. There are often many Hetalia - related fanfics there. Just use the search terms 'Hetalia', 'Russia', 'female America' and 'pregnant' to narrow down your search.
America might be initially shocked by the pirates' rowdy behavior. He could try to impose his own sense of order, but the pirates would just laugh at him. For example, when he tells them to line up for food, they might just push and shove like they always do.
Friendship is a very common theme. They often show how England and America have a bond that has developed over time, through shared history and experiences. For example, in many fanfictions, they start off with misunderstandings but gradually grow to understand and support each other.