A new, powerful villain could be the cause. This villain might target Ash's Pokémon and manage to steal them all away. Ash would be heartbroken not only because he lost his Pokémon but also because he feels he has failed to protect them.
Maybe it's a legendary Pokemon in disguise. In some fanfictions, legendary Pokemon often play important roles. It could give Ash the egg as a test or because it senses that Ash has a pure heart. Another possibility is that it could be a young trainer who can't take care of the egg anymore and trusts Ash to do a good job.
In most of these fanfics, the relationship between Ash and Sylveon is central. It often shows how Sylveon becomes a strong partner for Ash in battles. There are also elements of friendship and loyalty. For example, Sylveon might use its ribbons to protect Ash from danger. And there's usually a lot of description about Sylveon's cute and elegant appearance, as well as its powerful Fairy - type moves.
Team Rocket is an obvious choice. They could use some new, devious hypnosis device to control Ash for their own gain, like trying to steal all his Pokémon.
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.
No, it's not appropriate. Pokémon is a franchise aimed at a wide range of audiences including children. This kind of fanfiction involves adult and inappropriate themes that are completely out of sync with the family - friendly and positive image of the Pokémon world. It can also be seen as disrespectful to the original intention of the characters and the story.
According to the information provided, I recommend the novel " Doomsday Sharpshooter " to you. The male protagonist Deng Yun was bound to a gunner system on the eve of the doomsday. As long as he killed zombies, he would gain experience. However, he did not have a gun, so he began his zombie life with a slingshot. He could fly over roofs and walls. This novel has been completed. I hope you like my recommendation.😗
There are several fanfics out there with the 'ash gets a sylveon' theme. You might want to check out websites like FanFiction.net or Archive of Our Own. Just search for 'ash sylveon' on those platforms and you'll find a bunch. Some of them might have unique takes on how Ash acquires Sylveon, like through a special encounter in a mysterious forest or during a rescue mission.
One problem could be that it might change the perception of the original characters in an unwanted way. Fans who love the original relationship between Ash and Lillie might not like this new, more adult - themed take.