Well, it could be about Mr. Mime helping Ash's mom around the house in Pallet Town. Maybe Mr. Mime uses its psychic abilities to make gardening easier for her, or it could be a comical story where Mr. Mime tries to imitate Ash in front of his mom and gets into all sorts of silly situations.
Perhaps in the fanfiction, Mr. Mime becomes very attached to Ash's mom. It could start to mimic her every move and gesture. This annoys her at first, but then she realizes that Mr. Mime is just lonely. So, she starts to include Mr. Mime in more of her daily activities, like cooking and cleaning. Eventually, they form a strong bond that is based on mutual understanding and respect.
Maybe a disgruntled neighbor who is jealous of Ash's mom's relationship with Mr. Mime could be the villain. This neighbor could try to cause trouble between them by spreading false rumors or trying to drive Mr. Mime away. It would be a different kind of villain, more of a personal antagonist rather than a typical Pokémon - stealing bad guy.
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.
One possible plot could be that Professor Oak discovers a rare Pokemon that he believes Ash's mom might be interested in. He then takes it to her to show her, and they end up having a long conversation about Ash's adventures and how proud they both are of him.
Maybe Gary Oaks discovers a Mr. Mime that has an extraordinary power that's different from others of its kind. This makes Gary want to protect the Mr. Mime from those who might want to use it for wrong reasons. He then has to go through various challenges to keep it safe, and in the process, they form a deep connection.
One possible plot could be that his mom reveals she has been working with an evil organization all along. She hands Ash over to them for some dark purpose, like using his unique Pokémon connection for their evil experiments. Ash would feel extremely hurt and betrayed, having to then find a way to escape and also come to terms with his mother's true nature.
Perhaps the mime is in love with someone who doesn't notice him. He follows her around in his mime world, always a few steps behind. His movements are gentle and longing, like when he mimes opening a door for her but she just walks through without seeing him. His face shows a constant state of heartbreak as he tries to get her attention but fails every time.
Mr. Mime might start off as a shy Pokémon but as it spends time with Gary, it could become more confident. It could learn to use its powers more effectively in front of others, especially when helping Gary in difficult situations. Gary, in turn, would learn to rely on Mr. Mime more, strengthening their bond.
It could be about Ash and Gardevoir going on adventures together in the Pokémon world. Maybe they team up to face a powerful new villain.
It might be an adventure to an ancient ruin. Ash and his mom could be exploring the old, crumbling structures, looking for lost treasures. His mom might be teaching Ash about the history of the place as they search, and they could encounter some traps or puzzles along the way.
Well, they might have adventures exploring different regions. Ash could show his mom all the Pokémon he's caught over the years.