Well, it could be about Ash having daily adventures with legendary Pokemon. Maybe he stumbles upon a Mew in a forest and they start a friendship. He could learn about the unique powers and personalities of legendaries like Lugia and Ho - Oh in his everyday encounters.
In some fanfics, the life debt might lead Ash to gain special abilities. For example, if the Legendary Pokémon is Lugia, Ash could be given the ability to communicate with water - type Pokémon more effectively. This would not only help him in battles but also in understanding the ecosystem of the ocean better, all because of his life debt to Lugia.
Obviously, Ash would be one of the main characters. His Pikachu would also play an important role as his trusty sidekick. And then, there could be various legendary Pokemon like Celebi, who might befriend Ash and guide him through different adventures in the Pokemon world.
One common theme is sacrifice. Ash might have to sacrifice his own goals or dreams temporarily to fulfill his life debt. For example, he may give up a chance to compete in a big Pokémon tournament to help the Legendary Pokémon in need.
They would probably continue their Pokémon adventures together. Maybe they'd start a Pokémon training school. Ash could teach battling techniques, and Misty could teach about water - type Pokémon care.
In some fanfictions, Ash can touch Pokémon to show a deeper bond. It might be a way for the author to create more emotional and physical interactions between Ash and his Pokémon pals, different from the normal anime where physical contact like hugs is not as common.
Ash would likely be shocked at first. He's so used to seeing the good in people, especially fellow trainers. But then he'd probably get angry and determined to set things right. He has a strong sense of justice, so he wouldn't let the betrayal go unpunished.
Some fanfictions might have Misty betray Ash. Perhaps she gets lured by the promise of power or some valuable reward. She could be tricked into believing that Ash is no longer the friend she thought he was and that betraying him is for the greater good.
Ash might meet a strict teacher who has no idea about Pokémon at first but gradually becomes interested in them. He could also meet a rival in school who challenges him not only in academics but also in Pokémon battles.