In many expelled Harry Potter fanfictions, the Weasley family as a whole gets involved. Molly Weasley would be worried sick about Harry and would be cooking up plans to get him back. Fred and George might use their prank - making skills to expose the truth behind the expulsion. Ginny, being Harry's love interest in the original series, would be actively involved in trying to clear his name. And don't forget about the Hogwarts professors. McGonagall would probably be torn between following the rules and believing in Harry.
One possible plotline could be that Harry is wrongly expelled due to false accusations by a dark wizard who has infiltrated Hogwarts. Harry then has to prove his innocence while on the run from the Ministry. Another plotline might involve Harry being expelled because of a new, strict and unjust headmaster who has a vendetta against him. He could then start his own school for outcast witches and wizards.
He might be extremely angry and feel a great sense of injustice. He has always seen Hogwarts as his true home, and being expelled would be like losing his family. So he would probably fight back, trying to prove his innocence and get his expulsion reversed.
One possible reason could be that he is wrongly accused of using dark magic. In some fanfictions, the Ministry of Magic might be more corrupt or easily influenced, and they could frame Harry for something he didn't do, leading to his expulsion.
These fanfictions often allow for more character development outside the traditional Hogwarts setting. Harry, being expelled, is forced to grow in different ways, which can be really interesting to read about. He may face challenges he wouldn't have at Hogwarts and have to rely on his own wits more. For example, he might have to find new ways to practice magic or deal with the wizarding world's prejudice against an expelled student. This all adds depth to his character.
There might be a fanfiction where Harry is expelled due to political machinations within the wizarding world. For example, the Ministry of Magic wants to control him and uses false evidence to expel him. In the fanfic, Harry could end up leading a resistance against the corrupt Ministry, which would be an exciting twist on the original story.
A dark wizard or witch from outside Hogwarts could be the main antagonist. They might target Harry specifically because of his reputation as the 'Chosen One' and use their influence to get him expelled from Hogwarts so that he is more vulnerable and easier to attack.
Perhaps Harry accidentally unleashes a powerful and uncontrollable magic spell within the school grounds that causes a great deal of damage. Despite it being an accident, the Hogwarts administration, fearing further chaos, decides to expel him. Harry would then have to go on a journey to learn more about this magic and how to control it, all while trying to prove to Hogwarts that he is worthy of being readmitted.
In some fanfictions, Harry might be expelled for being wrongly accused of using dark magic. Maybe someone framed him by planting dark magic items in his dorm. Another reason could be that he continuously breaks school rules in a way that the school can't tolerate, like sneaking into the Forbidden Forest too many times without permission.
After being expelled, Harry's life takes a major turn. He might have to find alternative ways to continue his magical education. Maybe he seeks out some rogue wizards who are willing to teach him in secret. Socially, he could be ostracized by his former friends at Hogwarts who are afraid of getting in trouble for associating with him. He also has to deal with the disappointment of his mentors like Dumbledore, if they don't support his expulsion. His living situation might change too. He could no longer stay at Hogwarts, and might have to find a new place to live, perhaps with some relatives who are not as understanding as the Weasleys. This whole situation could make him more determined to prove his innocence or fight against the system that expelled him.