In this fanfiction, it might be that Latias controls Ash as part of an experiment. Latias is a curious Pokémon and wants to see how a human like Ash would react in different scenarios. So she uses her powers to make Ash explore areas he might not have otherwise, like a hidden and mysterious part of the Pokémon world. This could lead to Ash discovering new Pokémon species or even new types of Poké Balls. As he's being controlled, his initial fear turns into a sense of adventure as he experiences things he never thought possible.
The primary characters are Latias and Ash. But we can't forget about Misty and Brock if we consider the wider Pokémon universe. They might show up in the fanfiction, worried about Ash when they see his unusual actions under Latias's control. Also, there could be local Pokémon in the areas where the story takes place that interact with Ash while he's being controlled. These interactions could be either friendly or hostile, depending on the nature of the Pokémon and how they perceive Ash's strange behavior.
One possible plot could be that there is some sort of a prophecy in the Pokémon world that foretells the birth of a special baby from a human and a legendary Pokémon like Latias. Ash gets involved in this prophecy without knowing at first. As the relationship between Ash and Latias develops, they find out about the baby. The baby could have a great destiny, like being the bridge between different Pokémon species and humans. Ash would have to teach the baby about the values he believes in, like friendship and perseverance.
A plot could start with Latias being in some kind of danger, and Ash comes to her rescue. This leads to them spending more time together. As they get to know each other better, they realize their feelings are more than just friendship. After getting married, they might have to deal with people or Pokemon who don't approve of their relationship at first. But through their kindness and the strength of their love, they win them over and continue their happy life together.
Well, it could be that Latias has been influenced by an ancient prophecy. According to this prophecy, Ash is the key to unlocking a great power. But she can't just ask Ash to come with her as she fears he will be stopped by his friends or other Pokémon trainers. So, she decides to kidnap him. As time passes, Ash manages to communicate with Latias and understand her motives. He then decides to cooperate with her, not because he has to, but because he realizes the importance of the situation.
Perhaps in this fanfiction, Ash goes to the town in search of a rare Pokemon item. But when he arrives, he discovers that Latias and Latios are in trouble because of some evil forces. So, Ash decides to help them out. He uses his battle skills and knowledge of Pokemon to fight against these forces. Along the way, he also learns about the unique culture and traditions of the town.
Ash could catch Latias in a place full of ancient ruins. Latias might be guarding some sort of Pokemon relic there. Ash shows respect for the place and Latias' role. This makes Latias interested in him. Eventually, Ash catches Latias and they go on to discover more about the relic's power together.
Well, it could be about Ash somehow magically being transformed into a Latias. Maybe he then has to deal with the powers and instincts that come with being a Latias. He might try to find a way to turn back while also exploring the world from a new perspective as a Pokemon.
Perhaps in the fanfiction, the control isn't a malicious or intentional thing at first. Ash could be under the influence of a strange Pokemon's power. As a result, Serena starts acting in ways that are out of character. This confuses both of them at first. But as they figure out what's going on, they learn more about each other. Serena might discover that Ash has always wanted her to be more assertive in her Pokemon training, and Ash realizes that Serena has been holding back some of her true potential because of self - doubt. In the end, they find a way to break the control and Serena becomes a stronger trainer with a new - found sense of self.
One possible plot twist could be that Latios and Latias are actually under the control of a more powerful and evil Pokémon. Ash initially thinks he's caught them to protect them, but then discovers he has to fight against the real threat that has been manipulating them all along.
Well, it could be that Latias uses her psychic abilities to influence Ash's decisions in the story. Maybe she manipulates him into going on a particular adventure or making a choice that he wouldn't have otherwise made.