One plotline could be that Ash uses his ability to speak to Pokemon to solve a mystery. For instance, a group of Pokemon are acting strangely, and by talking to them, he discovers that there is a hidden force affecting them. Another could be that he has to convince a powerful and angry Pokemon not to destroy a village by communicating with it on a deeper level.
In this fanfiction, a plotline could be that Ash is called upon by the Pokemon League to use his unique ability to communicate with Pokemon. There is a problem with a large number of Pokemon going missing. By talking to the remaining Pokemon, he follows the clues and uncovers a dark organization that is capturing Pokemon for evil purposes. His ability to speak to Pokemon is crucial in rescuing the captured ones and stopping the organization.
One possible plotline could be that Ash somehow gets merged with Lugia's power during an unexpected encounter in a mysterious place. Maybe he's trying to protect Lugia from some evil force, and in the process, a magical transformation occurs. This could lead to him having to learn how to control Lugia - like powers, such as flying and using powerful water - based attacks. Another plotline could involve Ash being transformed into a Lugia - like form and having to hide it from his friends at first, fearing their reaction.
One possible plotline could be that Ash is forced into a dress as part of a prank by Team Rocket. They might use some sort of device to make him unable to take it off easily. Another could be in a different region where there's a strange cultural tradition that requires outsiders to wear a dress in a certain situation, and Ash, being unaware, gets caught up in it. Maybe his Pokémon try to help him get out of the situation in comical ways.
A plotline could involve a curse or a strange experiment gone wrong that causes Ash to be in a situation where he has to give birth. Or perhaps there's a parallel universe where the rules of Pokemon are different and male characters can give birth. Ash could be on a journey in this universe and have to deal with this new and unexpected situation, learning about himself and the world around him in a completely different way.
One possible plotline could be that it's set in an alternate universe where some magic or strange phenomenon causes them to become pregnant. Another could be a scientific experiment gone wrong scenario. But these are just wild speculations as such a concept is very far - fetched from the original Pokemon concept.
One possible plotline is that Ash is on a journey to prove himself against the toughest opponents and finally faces Brandon. He uses his Pikachu in a unique way to gain an edge in the battle.
Maybe Harrison comes to Ash's town and challenges him. Ash accepts, and during the battle, there are flashbacks of their past experiences and how they got to where they are as trainers. This could add depth to the story and show their growth.
One possible plotline could be Ash's discovery of his own latent legendary powers. Maybe he starts to exhibit abilities that are unique to legendary Pokemon, like controlling the weather or having an extraordinary bond with all types of Pokemon.
Well, it could change the story in many ways. For example, he can directly understand Pokemon's needs and feelings. So he might be able to prevent some Pokemon from getting into dangerous situations more easily. He could also form stronger bonds with his Pokemon team, leading to more effective battle strategies as he knows exactly what they are capable of and what they are thinking.
Pikachu is a very likely candidate. It's Ash's most iconic Pokemon and has a deep bond with him. Whenever Ash is in trouble, Pikachu often steps up to protect him.
In the fanfiction, perhaps there's a plot where Ash has to save a legendary pokemon from an evil organization. He might use his experience and the trust of his other pokemon to rescue it. This would not only show his courage but also how much he values all pokemon, whether they're common or legendary.