Pikachu would be a likely candidate. Pikachu is Ash's most loyal Pokémon and has a very strong bond with him. It would sense that Ash is in trouble and would go on a mission to rescue him. Pikachu could use its electric attacks to fight off Gary or any obstacles in the way.
Ash would probably be very confused at first. He might try to reason with Latias, asking her why she is doing this. If she doesn't respond, he would start looking for ways to escape. He would also be worried about his Pokémon and his friends, wondering if they are safe.
In this kind of fanfiction, perhaps Gary has some extreme jealousy towards Ash. Maybe he kidnaps Ash because he wants to prove himself superior in the Pokémon world. He could be tired of always being in Ash's shadow and thinks this is a way to get the upper hand.
A possible fanfiction could be about a young trainer who encounters Suicune and Entei in a vast, mysterious forest. The big pokemon are initially intimidating, but as the story progresses, the trainer discovers their gentle and protective sides. Maybe the trainer is on a quest to understand the ancient powers of these pokemon and forms a unique bond with them along the way.
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.
A fanfiction could be set in a post - apocalyptic pokemon world where Suicune and Entei are the last of the big, powerful pokemon. A lone survivor, who was once a pokemon trainer, finds them and tries to convince them to help restore the world. This story could explore themes of hope, friendship and the power of nature through the pokemon's abilities.
I haven't come across a fanfiction where Ash turns into May's mom myself. But given the creativity of fanfiction writers, it's entirely possible that someone has written such a story. It could potentially be part of a more complex plot within the Pokémon fanfiction universe, perhaps involving some sort of magic or strange experiment gone wrong that causes this transformation.
In some fanfictions, Ash gets angry. He might be angry at the world, at fate, for taking his mom away. This anger could manifest in different ways, like being more reckless in battles or snapping at his friends when they try to comfort him. But deep down, it's just his way of dealing with the overwhelming pain of losing his mother.
One possible plot could be that Serena's mom discovers Ash's true potential as a Pokemon trainer and decides to help him in his journey. Maybe she has some old Pokemon training techniques that she imparts to Ash, which helps him win a big tournament.
May would probably look up to Ash even more. Since Ash has a lot of Pokemon - related knowledge and experience, as her'mom', he could pass on some unique skills and wisdom. Their relationship might also have some emotional depth added to it, like when May is feeling down, Ash would be there to comfort her like a mother would, which is different from their usual trainer - to - trainer relationship.