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.
Well, Harry could marry one of the veelas. Given their beauty and allure, it would add an interesting dynamic to the story. Their relationship could involve Harry having to deal with the jealousy of other wizards and also the cultural differences of the veelas. Or he could marry a descendant of one of the founders of Hogwarts. This would tie in a lot of Hogwarts history into their relationship, and their wedding could be a grand event that unites different factions within the wizarding community who have ties to the founders.
Maybe Dumbledore, if he were still alive in the fanfiction. Dumbledore was highly respected and had a great influence. Knighting Harry would be a way for him to publicly recognize Harry's achievements and heroism on a grand scale.
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.