In some fanfics, there could be a magical creature or spirit that Mundungus has befriended in the past. This creature or spirit, being loyal to him, might try to help him in some way. It could be a house - elf that he once helped, and now the house - elf wants to return the favor. Or it could be a magical bird that can carry messages and tries to find someone to assist Mundungus in his predicament.
Well, he might face jail time in the context of the fanfic world. Maybe the wizarding authorities will confiscate his ill - gotten gains too.
There could be some old and wise wizards or witches from outside of Hogwarts who come to help. Maybe they heard about Harry's heroics in the battle and feel obligated to use their ancient magic knowledge to save him. And, of course, Ginny would be by his side, using her love and determination to will Harry back to health.
There could be some new characters introduced in the fanfic. Maybe a powerful wizard from a far - off land who has heard of Harry's plight and decides to help. Or perhaps a group of outcasts in the wizarding world who see Harry as a symbol of their own struggle against the unjust system and come to his aid.
He might have been put on trial. The wizarding court would then decide his fate based on the evidence presented against him. If the court was corrupt or influenced by those who feared Harry's power, he could be wrongly convicted.
Maybe it's a corrupt Ministry of Magic official. They might use Harry's credibility to their advantage and trick him into providing false information about Weasley. They could be after the Weasley family for political reasons or because they want to cover up their own misdeeds by creating a distraction with Weasley's arrest.
Ron and Hermione are the most likely ones. They are his best friends and have always been by his side through thick and thin. They would probably follow him without hesitation and use their own skills, like Hermione's vast knowledge and Ron's loyalty and practical skills, to assist in the ship - stealing adventure.
Dumbledore could potentially be a source of help. He is extremely powerful and wise. He could have given Harry some advice or a special magical item before the encounter with the dragon that would prove crucial in Harry's ability to kill the dragon.
His friend Ron could come back to help him. Ron is always loyal and might not want to leave Harry behind. He could bring his family's flying car again to pick up Harry.
Perhaps it was because of the power he amassed in the process of defeating Voldemort. In the fanfic world, power can be a very scary thing to those in authority. Harry's new - found and possibly overwhelming power could have made the existing magical power structure feel threatened. So, they arrested him as a way to control and contain that power, rather than seeing him as a hero who saved the wizarding world.
His friends like Ron and Hermione might help him. They are always there for Harry and if they believe that Harry has a valid reason for hiding from Dumbledore, they would likely do whatever they can to keep him hidden and safe. For instance, Hermione could use her intelligence to create spells or distractions to keep Dumbledore off Harry's trail.