Maybe Ash and his Pokemon travel to a parallel dimension where the rules of Pokemon battles are completely different. In this dimension, Pokemon can combine their powers in unexpected ways. Ash has to quickly adapt and learn these new battle styles in order to compete in a grand tournament there and find a way back to his own dimension.
A plot could involve a Pokemon that is cursed. This Pokemon joins Ash's team unknowingly carrying the curse. As they travel, strange things start to happen. Ash's other Pokemon start to lose their powers one by one. Ash has to find a way to break the curse, which involves a journey to a mysterious and dangerous place where an ancient Pokemon lives that has the knowledge to remove the curse.
One interesting plot idea could be Ash and Dawn teaming up to explore a new, uncharted region in the Pokemon world. They could encounter rare and unique Pokemon species that are not known to the rest of the world. Another idea is that they participate in a special tournament where the rules are very different from the regular Pokemon battles, like battles on floating platforms or with certain type restrictions. A third plot could be that they discover a mystery related to an ancient Pokemon civilization and have to solve it to save the present - day Pokemon world from some kind of threat.
One idea could be that Ash and Latias team up to save a group of Pokémon from an evil organization. Ash uses his battle skills and Latias uses her psychic powers.
One idea could be that Ash and Manaphy go on a journey to find a rare type of Pokemon food that can heal all Pokemon. Another could be that Manaphy gets lost and Ash has to traverse through a dangerous forest to find it. Or perhaps they team up to save a group of Pokemon from an evil trainer.
A more complex plot could be set in a world where there is a threat to all Pokemon. Ash and Astrid, along with their Pokemon friends, have to gather a set of ancient artifacts to save the Pokemon world. Along the journey, they have to overcome their own personal doubts and differences to succeed.
One plot idea could be Ash and Red teaming up to face a new, extremely powerful and mysterious Pokemon. They would have to combine their unique skills and knowledge of different Pokemon regions.
Maybe a fanfic where Ash and Brock discover a hidden Pokemon village that is in danger from a powerful, evil Pokemon. They must use their skills and the help of the village Pokemon to defeat the threat. This could involve Brock using his knowledge of Pokemon care to heal the injured village Pokemon, while Ash strategizes for the battle.
One idea could be Ash and Sabrina teaming up to face a new and powerful Pokemon that threatens their region. They have to combine their unique skills and knowledge of Pokemon to defeat it.
Well, a great plot idea might be that Red and Ash find a mysterious Pokemon that has powers neither of them have ever seen. They have to work together to figure out how to care for it and also protect it from an evil organization that wants to use its power for bad. Maybe they also learn some new training techniques from each other along the way.
One idea could be Ash facing off against old rivals who are jealous of his new Kalos champion status. They challenge him in unique battles that test his skills in new ways.
Perhaps a fanfiction where Lusamine's past experiences with Ultra Beasts lead her to seek Ash's help. Ash, with his unique bond with Pokemon, could assist her in finding a way to better understand and coexist with these mysterious creatures. This could involve a lot of character development for Lusamine as she learns from Ash's positive and empathetic approach to Pokemon.