Yes, there are many interesting Uta One Piece fanfictions. Some focus on exploring Uta's backstory in more depth, like her relationship with Shanks before she became a famous singer. Others might create alternate universes where Uta's powers are used in different ways to change the course of the One Piece story.
Probably not often. Marines are usually busy with their duties like maintaining order, fighting pirates, etc. They don't seem like the type to have a lot of time for reading fanfiction. But it's possible that some younger or more laid - back Marines might give it a try.
Well, it could be interesting. Marines in the One Piece world have strict rules and values. Reading fanfiction might expose them to different interpretations of their own characters. They could either find it amusing to see how fans view them or get annoyed if the fanfiction goes against their ideals.
One common theme is the exploration of Uta's true nature. Since she has a complex character in the movie, fanfictions often try to dig deeper into what makes her tick. Another theme is the power of her music, like how it can be a weapon or a tool for peace in different scenarios.
One idea could be Uta using her music powers in new and unexpected ways. For example, she might create a song that can temporarily freeze time in a small area within the One Piece world, allowing her to escape from a tough situation or pull off a clever heist. Another idea is a fanfiction where Uta teams up with the Straw Hat crew for a special mission. She could use her music to communicate with sea creatures and gain their help in navigating dangerous waters.
Their relationship could start with Uta being initially distrustful of Luffy. After all, Luffy is a pirate and Uta has her own ideas about pirates. But as she sees Luffy's pure heart and his determination to protect his friends and pursue his dreams, she starts to warm up to him. They might then become friends who share their different stories and experiences.