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.
If there was a fanfiction where Ash becomes May's mom, it could be a story full of growth and discovery. Ash would have to learn about the responsibilities of motherhood. He might start to understand May's struggles better from a mother's point of view. This could lead to a stronger bond between them. There could also be elements of mystery as to how and why this transformation occurred, and perhaps a quest to reverse it while still learning important life lessons along the way.
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.
It could be that in an alternate universe, Ash is born as a much older person and becomes May's mother in a more traditional sense. He might have different Pokemon knowledge and experiences which he passes on to May. He could be strict about some things like safety during Pokemon battles and lenient about others, like letting May choose her own first Pokemon even if it's not the most common or powerful choice.
In an 'Ash turns into Latios' fanfiction, perhaps Ash is turned into Latios as a way to better understand the Pokémon he has befriended. As Latios, he might be able to communicate with other Pokémon on a deeper level. For example, he could learn the secrets of legendary Pokémon that have always been a mystery. His friends, like Pikachu and Misty, would initially be shocked but then have to figure out how to help him reverse the transformation or support him in this new form.
One possible story could be that Ash is hit by a strange Pokémon attack that turns him into a Pichu. As a Pichu, he has to rely on his Pokémon friends in a whole new way. Pikachu, for example, might take on a protective role, guiding Ash - now a Pichu - through the challenges they face. And Ash would have to learn to control his new Pichu abilities like using Thunder Shock in a more instinctive way since he's now in a Pokémon's body.
In a fanfiction where Ash turns into a Vulpix, it could start with Ash exploring an ancient and forbidden area. There, he disturbs an old relic or power source that causes the transformation. As a Vulpix, he discovers that he has new abilities such as being able to use Ember and Quick Attack in a more instinctive way. His friends, like Misty and Brock, would be shocked but would quickly try to find a way to reverse the transformation. Meanwhile, Ash in his Vulpix form might make new Pokémon friends who are initially wary but then accept him as one of their own, leading to some interesting adventures in the Pokémon world.
In this Hoenn fanfiction, Ash and May could be on a quest to find a special item that is said to have great power. They face various obstacles and meet different characters along the way. Their different personalities - Ash being more impulsive and May being more level - headed - would play a big role in how they overcome these challenges.
The plot could involve Ash trying to get back to his normal form. He has to go on a journey as Dratini to find a way to reverse the transformation. Along the way, he meets other Pokemon who either help or hinder him. Maybe he has to face some challenges that are specific to his Dratini form, like fitting into small spaces or using his new abilities to cross obstacles.
Ash would initially be heartbroken. He has shared so many adventures with May, and this betrayal would shatter his trust. He might go off on his own for a while to think things through. When he comes back, he could be more determined than ever to prove himself without May. He might also be a bit more cautious when it comes to making new friends. His Pokémon would sense his distress and try to comfort him, but it would take a long time for him to fully get over May's betrayal.
Definitely their Pokémon friends. Pikachu, of course, would be there. May's Blaziken and Ash's Greninja would also be among the guests. Their human friends like Brock, Misty, and Dawn would surely come. And perhaps some of the Gym Leaders they've met during their travels would also attend, like Roxanne from Rustboro City.