Perhaps Ash is transported to a parallel Pokemon world where he is transformed into Lugia. Here, he has to find a way to return to his normal form and his own world. Along the way, he meets different versions of his friends who don't recognize him in his Lugia form. He uses his newfound powers to fight against the dark forces that are ruling this parallel world and tries to restore balance.
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.
Ash's personality could shift to be more protective. Lugia is known as a guardian of the seas, and in his transformed state, Ash might feel a stronger sense of responsibility towards other Pokemon and the environment. His competitive streak, which is a big part of his normal personality, could be toned down as he now has much greater power and a different role to play in the Pokemon world. He would also have to deal with the isolation that comes with being such a powerful and different being, which could make him a bit more introverted.
In many fanfictions where Ash catches Lugia, it often involves a great adventure. Maybe Ash stumbles upon Lugia in a hidden cove. Lugia, being a powerful and majestic Pokémon, would be a huge addition to Ash's team. It could change the dynamic of his battles completely. He might have to prove himself worthy to Lugia first, perhaps by showing his kindness and determination in protecting other Pokémon in the area.
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.
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 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.
Another storyline could involve a female Lugia being in trouble because some evil Pokémon trainers are trying to capture it for its power. Ash hears about this and teams up with his friends to protect the Lugia. Along the way, they face many challenges and Ash has to use his battle skills and knowledge of Pokémon to outwit the villains.
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.