A great fanfiction could be 'When May Finds Ash'. This might tell a story where May finds Ash at a crucial moment, perhaps when he's in trouble or lost. Their relationship then takes on new dimensions as they try to find a way out of the situation together, relying on each other's skills and the bond they form during this time.
You can try fanfiction websites like FanFiction.net or Archive of Our Own. These platforms have a large collection of Pokemon fanfictions, and you can use the search function to look specifically for ones where May finds Ash.
A possible fanfiction plot could be that Ash, Serena, and May are invited to a mysterious island full of unknown Pokemon. They have to work together to survive and discover the secrets of the island. Ash's leadership, Serena's ability to communicate with Pokemon, and May's quick decision - making skills would be crucial.
In this fanfiction, Ash and May could be on separate adventures in Kalos at first. But then, they cross paths when they both get involved in a mystery related to a rare and powerful Pokémon in Kalos. They have to put their heads together, along with their Pokémon, to solve the mystery and protect the Pokémon from those who would use it for bad purposes.
In some fanfictions, Ash might develop a special bond with both Serena and May. For example, they could be exploring a new region together and encounter a rare Pokemon. Serena could use her knowledge of Pokemon behavior to track it, May could come up with a battle plan, and Ash would be the one to catch it. This kind of adventure would strengthen their friendship and show how well they work together.
It could be a rival trainer. Rival trainers are often very competitive in the Pokémon world. Maybe this rival is jealous of Ash and May's progress and decides to betray them to slow them down. They might spread false rumors about Ash and May, causing other trainers to be distrustful of them.
In this fanfiction, Ash could face a lot of challenges while taking care of the Pokémon egg. There might be other trainers or Pokémon who try to steal the egg. He would have to protect it at all costs. Also, he could meet some new friends who help him with the egg. They might give him advice on how to keep the egg warm and safe. And when the Pokémon finally hatches, it could form a really strong bond with Ash right from the start.
It could be a Pikachu. Since Pikachu is often associated with Ash and could be considered a special kind of 'first' pokemon for him in a different take of the story.
It could be an Eevee. Eevee is a very common yet special Pokemon. It has the potential to evolve into many different forms, so it would be an interesting addition to Fem Ash's team.