In the fanfiction, it might be that Misty has some sort of ulterior motive for stealing the Pokemon. Maybe she's being controlled by an evil force or she thinks it's the only way to protect the Pokemon from something worse. But still, Ash would not easily accept her actions and would likely fight to get the Pokemon back to its rightful place.
Well, they might have a friendly chat about their past adventures. Maybe they'll share stories of the Pokémon they caught and the battles they fought together.
Ash would probably be very shocked and hurt. Pokemon are like his family, and for Misty to steal one would go against everything he stands for.
Another possibility is that Ash wins the battle. Even though Misty has a new Pokemon, Ash's experience and the strong bond he has with his Pokemon could give him the edge. He might be able to quickly analyze the new Pokemon's weaknesses during the battle and use his Pokemon's attacks effectively to secure the victory.
Well, if Ash turns into a Pokemon in a fanfic, he might gain new abilities. For example, he could use Pokemon moves like Pikachu's Thunderbolt. He'd also have a different perspective on the Pokemon world. Maybe he'd understand their language better and be able to communicate with them more easily. This could lead to new adventures where he teams up with other Pokemon in ways he couldn't before.
Well, he might be really confused at first. Maybe he'd start to notice strange things about the world around him that don't quite fit the normal Pokemon universe he knows. He could see people acting in ways that seem more like how fans write in fanfictions.
Perhaps he'll share his new adventures and the knowledge he's gained from traveling different regions. He could inspire the new generation of Pokémon trainers in Pallet Town to aim for greater things.
Well, she might take the egg to a Pokemon Center first. The Nurse Joy there could give her some advice on how to take care of it. Maybe she'll start to feel a special connection with the egg right away.
He could find that his relationship with his Pokemon changes. Some Pokemon might be more protective of him now that he's in a more vulnerable form. Others might see him as less of a trainer and more of an equal. Ash would also have to deal with how the Pokemon world views hedgehogs. Are they seen as pests or something more interesting? This could lead to different reactions from different Pokemon species. And as he travels, he might discover new places that are more suitable for a hedgehog - like small burrows or areas with lots of bushes where he can hide and rest.
It's also possible that the hitman is really good at his job and manages to get close to Ash without being detected. But just when he's about to make his move, one of Ash's Pokémon that he didn't expect, like Pikachu who is always full of surprises, manages to fend off the hitman and save Ash.
The story could start with Ash being influenced by the love and commitment he sees around him in the Pokemon world. He starts to see Misty in a new light, not just as a friend and fellow trainer. As he plans the proposal, he might recall all their shared memories, from their first meeting to their latest adventures. He could take Misty to a place filled with their memories, like an old Pokemon gym they once visited together, and there, with heartfelt words, propose to her.