Another possibility is a Celebi. Celebi is a time - traveling pokemon. Ash could be led to Celebi by some strange time - related events. It would be a challenging catch as Celebi can manipulate time to avoid capture. But with Ash's experience and the unique skills of his pokemon, he might be able to succeed.
The fanfiction might focus on how Ash discovers the unique habits and weaknesses of the rare pokemon. He could meet some new friends or rivals along the way who also have their eyes on the same pokemon. And in the end, through determination and his bond with his pokemon, he manages to catch it.
Well, it could be about Ash traveling to a remote area, like a hidden forest or a mysterious island where the rare pokemon is rumored to live. He might face many challenges on his way there, like tough terrains or other aggressive pokemon. Then, when he finally encounters the rare pokemon, it could be a really intense battle where he has to use all his skills and knowledge of pokemon to catch it.
Well, in many fanfictions, Ash might have some special abilities or items. Maybe he gets a super - powerful pokeball or has a unique bond with the legendary pokemon that allows him to catch them more easily.
The plot could involve Ash hearing rumors about Mewtwo's location. He sets out on a journey to find it. Once he does, he tries to communicate with Mewtwo. Mewtwo initially resists but Ash's pure intentions start to sway it.
A plot might be that Lugia is being hunted by an evil organization. Ash discovers this and decides to protect Lugia. He uses his Pokémon to fight off the bad guys. In the process, he forms a bond with Lugia. Eventually, Lugia realizes that it's safe with Ash and allows itself to be caught. This would show Ash's heroism and his love for Pokémon.
One possible plot could be that Ash stumbles upon the shiny pokemon by pure chance while exploring a new, uncharted area. Maybe he's looking for a specific type of pokemon for a local competition and instead, he discovers this rare shiny pokemon. Another plot could involve the shiny pokemon being in danger from a group of poachers, and Ash has to rescue it. Then, after rescuing it, he forms a special bond with the shiny pokemon which is different from his bonds with his other pokemon.
He could develop more patience. Since rare pokemon are often hard to find and catch, he would need to wait for the right moment. This could also teach him to be more observant of his surroundings and the behavior of pokemon. And his bond with his existing pokemon could also strengthen as they work together to catch the rare one.
One possible plot is that Latias is injured and Ash finds it. He nurses it back to health, and during this process, they form a bond. Latias then decides to join Ash's team. Another plot could be that there is a tournament where the prize is a special item that can attract Latias. Ash enters the tournament, wins, and uses the item to catch Latias.
One possibility is that Ash catches Machop during a special event. For example, there could be a local Pokemon tournament where Machop is a prize for the winner. Ash participates and wins, thus getting Machop. Machop then quickly becomes part of Ash's team and starts training with the other Pokemon.
Another possibility is that Ash wins the battle. Even though Misty has a new Pokemon, Ash's experience and the strong bond he has with his Pokemon could give him the edge. He might be able to quickly analyze the new Pokemon's weaknesses during the battle and use his Pokemon's attacks effectively to secure the victory.