You can start by looking on fanfiction websites like Archive of Our Own or FanFiction.net. They have a large collection of various fanfictions and there's a good chance you'll find some about Marco the Phoenix there.
One great 'Marco the Phoenix' fanfiction could be about his adventures before joining a certain group. It might explore his training and how he developed his unique phoenix - like abilities.
Well, he might be used as a bargaining chip against the Whitebeard Pirates. His captors could try to extract information about the pirates' secrets or hidden treasures.
One interesting aspect could be the new powers Harry might gain. Phoenixes have the ability to heal with their tears and rise from the ashes. So in the fanfic, Harry could use these powers in unique ways, like healing his friends or himself in difficult situations.
I'm not sure of a specific 'harry has a phoenix form fanfic' off the top of my head. But you can try searching on popular fanfic websites like Archive of Our Own. They have a large collection of Harry Potter fanfics and you might be able to find one there.
Another possibility is that a rival pirate group that has a long - standing feud with the Whitebeard Pirates managed to finally get the upper hand and capture Marco. They could have been studying his movements for a long time and found a weakness in his defenses. This group might be led by a very cunning and powerful pirate captain who has a personal vendetta against the Whitebeard Pirates and sees capturing Marco as a major victory.
Star would be a likely candidate to rescue Marco. She has strong magical abilities and a deep friendship with Marco. She would stop at nothing to find him and bring him back safely.
The Chronicle of Marco Polo was a travel diary written by the Italian traveler, Marco Polo, describing his experiences in China, Mongolia, Siberia, and Europe. However, there is no explicit mention that it was written by Marco Polo.
The richest city in the world described in the Chronicle of Marco Polo referred to Beijing in China. In his diary, Polo described the glorious history of Beijing and its magnificent buildings such as the Forbidden City, the Temple of Heaven and the Summer Palace, which were still regarded as treasures of world culture.
The Travels of Marco Polo was originally recorded by the Italian traveler, merchant, and scholar Marco Polo in his diary. He traveled to Europe for nine years from 1282 to 1283 and described his travel experiences and experiences in detail in his diary.
Later, this travel note was adapted into novels, movies, TV series, and other works many times. Among them, the more famous ones were Faulkner's novel To the Lighthouse (The Feynman Feynman's autobiography mentioned that he had heard Faulkner read this novel) and some chapters from the American science fiction novel Base by Asimoff were used as the material for the novel.