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 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.
A plotline could involve a special Pokemon that both Ash and Paul are interested in. They might compete to catch it, which could involve a series of challenges and battles. Or, perhaps there's a new type of Pokemon training technique that one discovers and the other tries to learn from or outdo. This could create an interesting dynamic in the fanfic.
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 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 a fanfiction, it really depends on how the author wants to write it. If the author wants to show Ash's growth, he might have Ash use some new strategies. Maybe Ash would use his other Pokemon like Charizard in a really creative way to outwit Lance.
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.
There could be such fanfictions. One possible plotline might be that Paul, in a self - sacrificing moment, saves Ash from a powerful and dangerous Pokemon attack. Maybe Paul realizes the importance of Ash's journey and his pure heart for Pokemon, and decides to put his own life on the line. For example, if there's a rampaging legendary Pokemon that Ash is trying to calm down, Paul jumps in to protect Ash when the situation goes awry.
Ash could win. He has a lot of experience and strong Pokémon. His Pikachu is super powerful and has defeated many tough opponents. So, it's likely that Ash would come out on top in this fanfiction battle.