You can try fanfiction websites such as FanFiction.net or Archive of Our Own (AO3). They have a large collection of fan - made stories, and you can easily search for 'Pokemon Ash grows up fanfic' using their search functions.
One popular 'Pokemon Ash grows up fanfic' could be about Ash becoming a Pokemon Master and how he manages the responsibilities that come with it. For example, he might have to deal with politics in the Pokemon world regarding training regulations.
Team Rocket is the most likely candidate. They've always been after Ash's Pikachu and tying him up would be a way to stop him from protecting his Pokémon. They could use ropes or some kind of Pokémon - powered restraining device.
One possible plot could be that Ash gets tied up by a group of Team Rocket grunts who manage to outwit him. They tie him up to stop him from interfering with their latest evil plan to steal rare Pokémon. His Pokémon then have to work together to free him and foil the villains' scheme.
There could be a young trainer who looks up to Ash and follows him around to learn from him. Another might be a rival from a far - off region who has a very different training style.
For Ash, Pikachu is always a great choice as it's his most loyal and powerful partner. Alongside that, a strong flying type like Charizard could be useful. For Brock, his Onix would be a staple. And perhaps a water - type like Vaporeon could be good for him to balance his team. Since they are teaming up, having a psychic - type like Alakazam could also be beneficial as it can help in battles and also with communication in tricky situations.
One idea could be Ash becoming a renowned Pokémon master and having to deal with the new responsibilities that come with it, like mentoring new trainers. Another could be him facing a new and powerful villain who targets him specifically because of his fame.
I'm not sure about its popularity. It might be popular among some fans who like to see alternative and edgier takes on the Pokemon story. But it could also be quite controversial as Ash is such an iconic and generally well - liked character, and making him a cheater goes against the common perception of him.
In Ash's adventures in legendary Pokemon fanfics, he often has extraordinary encounters. For example, he might form unique bonds with legendary Pokemon that are not part of the main canon. These fanfics can take his story in new and exciting directions, like having him team up with a Mewtwo in a way that defies the normal Pokemon - trainer relationship. It's all about exploring the untapped potential of his character in the context of the amazing world of legendary Pokemon.
One of the popular fanfics where Ash calls the strongest Pokémon could be those where he summons Arceus. In some fanfictions, Ash might have a special connection with Arceus, the so - called 'God of Pokémon'. This could lead to epic battles and adventures as he teams up with the most powerful being in the Pokémon universe.