Perhaps in this fanfic, the Ministry of Magic is under new management that is more closely allied with the Muggle government. Harry Potter and his friends are then given special tasks by the Queen (or her representatives) to use magic to improve the country. This could include using magic to clean up polluted areas, or to help in times of natural disasters. At the same time, they have to deal with internal wizarding politics that might oppose such close cooperation with the Muggles.
Obviously, Harry Potter himself would be a main character. He has the experience and the leadership skills from his battles against Voldemort. Hermione Granger would also be important as she has the knowledge and intelligence to come up with strategies for using magic for the country. Ron Weasley could be there too, providing the loyalty and the comic relief. And perhaps some new characters from the Muggle government side who interact with the wizarding world characters.
Well, it could be that Harry Potter, while on a magical mission, somehow gets transported to Star City where Oliver Queen operates. Oliver, being the vigilant Arrow, at first mistakes Harry for a threat. But then they realize they both fight for justice in their own ways. Harry could use his magic to assist Oliver in taking down some really tough villains that the Arrow alone couldn't handle, like some dark magic - infused criminals.
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.
Well, it could be that in the fanfic, Harry Potter uses his magic skills to protect the Queen from dark wizards who are plotting against her. Maybe he creates a powerful protective shield around the palace.
One possibility could be that in an alternate magical universe, Harry Potter, after all his adventures at Hogwarts and defeating Voldemort, travels to a different realm where he meets a queen. Maybe the queen is facing some magical threats in her kingdom and Harry, with his powerful magic, offers to help. As they work together, they fall in love and decide to get married. Their union could then bring about a new era of peace and cooperation between the wizarding world and the queen's non - magical kingdom.
Well, it might be that Harry Potter is accidentally transported to a non - magical realm where he is in trouble because he doesn't understand the Muggle world completely. The Queen, who has some knowledge of magic or is in contact with a secret magical society, discovers Harry and saves him from the perils of this strange new world for him.
Perhaps in this fanfiction, Harry Potter is recruited by the queen to solve some magical mysteries within the palace. He could use his knowledge of the wizarding world to uncover secrets that are plaguing the monarchy. For example, there might be a cursed object in the palace that is causing strange happenings, and Harry is the only one who can find and break the curse.
One possible plot could be that the Queen is in danger from dark wizards who want to take over the Muggle government. Harry, using his skills from Hogwarts, discovers the plot and uses magic to protect the Queen. He might use spells like Protego Maxima around Buckingham Palace.
Well, it could be about the Queen of England being a secret witch and having a connection to the wizarding world of Harry Potter. Maybe she helps Hogwarts in some way during a crisis, like providing a safe haven on the castle grounds that are protected by magic and Muggle secrecy charms at the same time.
Well, perhaps Harry Potter could use his magic to solve some sort of mystery in the Queen's palace. Maybe he'd help protect a valuable magical artifact that was in the Queen's possession without her knowledge of its true nature.