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.
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.
One possible plotline could be that Ash is betrayed by his friends. This heartbreak and sense of abandonment turns him evil. He then starts using his Pokémon in cruel ways to seek revenge on those who wronged him.
One possible plotline could be Ash discovers his latent Aura Guardian abilities gradually. He might start having strange visions or experiences related to aura. Then, he has to find someone to train him, like an old Aura Guardian who has been in hiding.
One possible plotline could be that Ash discovers May has been secretly working with a rival trainer in order to gain an advantage in a big tournament. This would make Ash feel betrayed as he thought they were friends and training partners. Another could be that May betrays Ash for a rare Pokémon that Ash was also after, choosing the Pokémon over their friendship.
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.
One possible plotline could be that Hermione travels back in time and becomes Harry's mother. This would involve a lot of time - travel magic and dealing with the consequences of changing the past.
Maybe in the fanfiction, Ash is given a Chespin by a mysterious trainer. This Chespin has some special powers that Ash has to figure out how to unlock. As they train together, they form a deep bond and go on to compete in a big Pokémon tournament where Chespin's unique abilities give them an edge.
Another plotline might involve Ash teaming up with some of the Team Rocket members, perhaps reluctantly at first. They could be forced to work together due to a common threat to both the good guys and the bad guys in the Pokemon universe. This would lead to some interesting character developments for Ash as he has to deal with the not - so - good nature of Team Rocket.
One possible plotline could be that Ash captures Suicune in an unexpected way, like during a special event in a new region. Then he has to deal with the consequences of having such a powerful and rare Pokémon, like other trainers being jealous or wanting to challenge him for it.
A plotline might be that Latios is from a different dimension or time, and it has come to Ash's world for a specific purpose. Ash gets involved in this mission, and together they have to face powerful enemies. This could introduce new Pokémon species from other dimensions and new battle strategies that Ash has to learn to keep up with the challenges.