Well, one possibility is that Ash wakes up with all the powers of Mew. He could then use these powers to quickly become the best Pokémon trainer ever. He might be able to communicate telepathically with all Pokémon, understand their needs and desires instantly, and use his newfound psychic abilities to battle more effectively. For example, he could levitate his Pokémon during battles, giving them an unexpected advantage.
In this kind of fanfiction, Ash might wake up with new Pokemon - like abilities. For example, he could communicate with Pokemon in a more direct and profound way, like understanding their every thought and emotion just by looking at them. He might also find himself physically changed, perhaps having some physical features of a Pokemon, which could lead to all sorts of interesting adventures and interactions with his friends and other Pokemon trainers.
She might try to prevent all the tragedies that befell her family. For example, she could warn Ned Stark about the Lannisters' schemes, which might change the whole course of events in Westeros. Maybe she would also use her knowledge of future alliances and enmities to her advantage, forging new relationships and breaking old, harmful ones.
At first, his friends might not believe it's Ash. But when he starts to show his usual personality traits, like his determination and kindness, they would start to accept it. They would then start to think about how they can use Ash's new form to their advantage. For example, they could use his ability to fly as Mew to reach places they couldn't before, or his psychic powers to sense danger from afar.
In this kind of fanfiction, one possible scenario is that Ash forms a special bond with Mew. Mew, being the mythical and playful Pokemon, could lead Ash on an adventure full of surprises. Maybe it shows Ash some hidden powers within Pokemon that he never knew before. This could help him in his battles and his journey to become a Pokemon master.
Perhaps he wakes up to find that the laboratory is in the middle of a big experiment gone wrong. There are strange creatures or phenomena around him, and he has to figure out how to escape and also stop whatever bad thing is about to happen. He might use his Pokémon knowledge and skills to get through this tough situation.
In the fanfiction, when Ash wakes up in the room, he might discover that he has new, unexpected powers. The room could be a sort of testing ground for him. He'll have to learn to control these powers while also dealing with the mysteries of the room, like hidden doors and cryptic messages on the walls.
The human might find themselves in a strange and mysterious underground world filled with unique creatures. Maybe they'll encounter a group of underground dwellers who have their own culture and language, and they'll have to figure out how to communicate and survive among them.
In the fanfiction, the person waking up as Lucy would have to face the challenges that Lucy herself has. This includes dealing with her past, like her relationship with her father. She would also have to manage her relationships with other guild members, such as Natsu and Erza. And of course, she'd need to keep up with her training to become stronger in magic. Maybe she'd even try to create new spells or improve on Lucy's existing ones.
In the fanfiction, they may decide to explore uncharted regions. With their combined knowledge and skills, they would be a formidable team. Along the way, they could discover new Pokémon species and form alliances with local Pokémon trainers. Their marriage would be strengthened by these shared adventures, and they would become legends in the Pokémon world for their discoveries and contributions.
Latias could initially go into hiding, feeling hurt and betrayed. She might avoid Ash at all costs. Then, Ash could realize his mistake and start searching for her to apologize.