Well, one possible plot could be that Bill takes Harry under his wing after some kind of danger at Hogwarts. He might teach Harry some advanced magic that he learned during his time as a curse - breaker. For example, Bill could show Harry how to deal with ancient and powerful dark magic spells.
Maybe Bill becomes Harry's guardian because of some family emergency within the Weasley family. Bill then decides to train Harry in combat magic, not just for self - defense but also to prepare him for a bigger battle against the dark forces. He could introduce Harry to some of his contacts in the wizarding world who are experts in different fields of magic.
One possible plot could be that Bill mentors Harry in the art of ancient magic. Since Bill has been around for a while and has dabbled in many different areas of magic, he could introduce Harry to spells and magical knowledge that has been long forgotten. This could help Harry in his future battles or in his general understanding of the magic world.
There could be some new characters as well. For example, some of Bill's old curse - breaker colleagues. They could bring in new elements of magic and danger. Maybe there's a mysterious wizard who has a vendetta against Bill and sees Harry as a way to get to him, which would make this wizard an important part of the story.
Perhaps in the fanfiction, there is a magical tournament being organized that requires the skills of all three. Harry is known for his courage and spell - casting abilities, Bill for his worldly knowledge and dueling skills, and Charlie for his physical strength and ability to handle dangerous magical beasts. They team up to compete and face various challenges along the way.
In 'harry potter the queen guardian fanfiction', it may show the Queen as a guardian of magical creatures. She could be in charge of protecting thestrals, for instance. Thestrals are quite unique and important in the Harry Potter world. The Queen might be using her influence and some special magical abilities to keep their habitats safe. This could also involve her having to deal with poachers or those who want to use thestrals for dark magic. Harry Potter and his friends could get involved in this situation too, either by accident or because they are asked to help by the Queen.
In some fanfictions, Fleur might leave Bill for Harry because she realizes that Harry has a certain selfless quality that she admires more. Harry's constant fight against evil and his willingness to sacrifice himself could be very appealing to Fleur. Also, it could be that the fanfiction is set in a situation where Bill is involved in something that Fleur morally opposes, and Harry represents a more morally upright choice for her.
Well, perhaps they could become friends quickly. Bill might introduce Harry to some interesting magical things he hasn't seen before, like some unique creatures. And they could start having little adventures together around the wizarding world.
One possible plot could be that Harry discovers Death as his guardian early on. Maybe Death protects him from Voldemort's initial attacks in a more direct way than in the original story. For example, when Voldemort tries to kill Harry as a baby, Death could appear as a shadowy figure and deflect the Killing Curse in a more obvious display of power.
The plot could develop with Harry and Bill trying to figure out how to handle the situation. They might have to deal with the reactions of the wizarding community. For instance, there could be a lot of gossip and rumors flying around, which they would need to face together.
In a 'Harry adopted by Bill fanfiction', perhaps the story begins with Bill returning to England and finding out about Harry's situation. He realizes that Harry needs a more stable and loving family environment. So, he starts the adoption process. Harry then gets to learn about Bill's work as a curse - breaker from up close. They might also face some opposition from the Ministry who are skeptical about Bill's ability to take care of Harry, but they prove everyone wrong in the end.
A potential guardian could be Sirius Black's spirit. Even though he died, his love for Harry could be so strong that it allows his spirit to act as a guardian. He could appear to Harry in times of need, give him advice, and even use his limited spectral powers to protect Harry from minor threats.