In fanfiction, the Dursleys could get arrested for child abuse. They treated Harry Potter horribly, locking him in a cupboard, underfeeding him, and constantly berating him. Another reason might be for illegal use of magic items. If they got hold of some magical objects through Harry and misused them, that could lead to their arrest.
One possible plot could be that Harry Potter finally has the courage to report the Dursleys' abuse to the proper authorities. Wizards from the Ministry of Magic, after getting involved, find evidence of the emotional and physical abuse Harry endured over the years. The Dursleys are then arrested and Harry is placed in a more positive and caring environment within the wizarding world, perhaps with the Weasleys.
It might be because of her anti - Muggle - born policies. Fanfiction often explores the idea that Harry discovers Umbridge's discriminatory plans against Muggle - borns. This is completely against the values of the wizarding world, and Harry, having Muggle - born friends, would take steps to expose her and get her arrested for such bigotry.
Well, it could be that he has a long criminal record and has been involved in various illegal activities like theft, fraud, and assault. Maybe he was part of a criminal gang that was into multiple kinds of crimes.
Obviously, Harry Potter would be a main character. He is the victim of the Dursleys' abuse and his story would be central to the fanfiction. The Dursleys themselves, Vernon, Petunia and Dudley, would also be main characters as they are the ones being arrested. And there might be some wizarding characters like Dumbledore or members of the Ministry of Magic who get involved in the case.
If the Dursleys got arrested in fanfiction, Harry's life would change drastically. He would no longer have to endure their abuse. He might be placed in a more positive environment, perhaps with the Weasleys or other friendly wizards. This would also mean that the Dursley - related subplots in the story would end, and new ones could start focusing on Harry's new situation.
Perhaps in fanfiction, Harry runs away from the Dursleys because he discovers a new threat that the Dursleys won't protect him from. He realizes that he has to take matters into his own hands and leaving is the only option. Also, he could be running away because he has made some new friends in the wizarding world who offer him a better alternative to living with the Dursleys.
Often it could be the magical community. Since the Dursleys have mistreated Harry, a wizard, for so long, the Ministry of Magic or some powerful wizards might decide to teach them a lesson. For instance, Dumbledore could use his power and wisdom to make sure they face consequences.
Yes, in many fanfictions, the Dursleys face consequences for their mistreatment of Harry.
In a fanfiction, Natsu might get angry if he was wrongly accused of something. Say, if he was blamed for a crime he didn't commit in the guild or in the magical world at large. He's an honest person, and being falsely accused would be a huge blow to his pride. Another possible reason is if a magic - based experiment went wrong because of someone else's carelessness and it put his friends in danger. Natsu values the safety of his comrades, and such a situation would make his blood boil. Also, if someone were to try and break up the Fairy Tail guild by causing internal disputes, Natsu would be enraged as he loves the unity and spirit of the guild.
There could be several reasons. Maybe the website is experiencing technical issues. It might be under maintenance or having server problems. Another possibility is that there are copyright issues preventing the release. Some fanfiction may violate the original work's copyright terms, so it's not made available.