Pikachu would be super excited. It has been with Ash for so long and would see Misty as part of the family. It might play more with Misty's water - type Pokémon. Misty's Starmie could be very protective of the new family unit and use its psychic powers to help around the house sometimes.
Ash's other Pokémon like Bulbasaur and Charmander would respect Misty as Ash's partner. They would probably learn new things from Misty's Pokémon. For instance, Bulbasaur could learn how to better adapt to wet environments from Misty's water - type Pokémon. Misty's Goldeen might be a bit shy at first but would soon get used to Ash's energetic Pokémon. Overall, their Pokémon would create a big, happy family together.
Misty would probably be shocked at first. After all, she and Ash have been friends for so long. But then she would be overjoyed. She might blush deeply and have tears of happiness in her eyes as she says yes.
Pikachu, Ash's main Pokémon, would be very confused. It's used to Ash's mannerisms and commands. When Serena is in Ash's body, Pikachu might not respond as well at first. It might keep looking at 'Ash' quizzically, wondering why he's acting so strange. Serena's Pokémon, like Braixen, might also be confused. They might sense that something is off with 'Serena' (Ash in her body) and be a bit hesitant to follow commands as usual.
The Pokémon League would be in shock. They would see Ash and Misty as traitors and would probably send out teams to capture them. Pokémon trainers who looked up to Ash would be very disappointed. They might start to question their own values and the values of the Pokémon training world.
At first, his Pokémon would be startled. They've always known Ash as a human. But then, they would start to adapt. Some Pokémon, like Squirtle, might try to play with Ash in his new form, thinking it's a new kind of game. However, his more loyal Pokémon, like Greninja, would be determined to find a way to turn Ash back. Greninja might use its special bond with Ash to try and communicate with him on a deeper level in this new form, to understand what happened and how to reverse it.
Yes, there are many fanfictions out there that depict Ash and Misty getting married. Some fans love to explore this pairing and create their own stories about their relationship evolving into a romantic one and leading to marriage. It's a popular concept among Pokemon fans who have been shipping Ash and Misty for a long time.
On Ash's side, his mom would be extremely happy and would start planning the wedding immediately. She would love Dawn like her own daughter. Ash's other friends like Tracey would send their best wishes. For Dawn's family, they would see Ash as a great match for Dawn. Her mother would be glad that Dawn has found someone who shares her love for Pokemon. Her father would be impressed with Ash's achievements in the Pokemon world and would wholeheartedly approve of the marriage.
One common theme is family. Writers often explore how Ash and Misty would start a family and raise their children in a world full of Pokémon.
Yes, there are quite a few out there. You can find them on various fanfiction websites. Some might focus on their married life adventures, like exploring new regions together with their Pokémon.
Some of his Pokémon might be very confused and sad. Pokémon like Pikachu, who has been with Ash for a long time, would probably miss the adventures and battles they had together. Pikachu might try to cheer Ash up and convince him to change his mind.
Ash might be heartbroken at first. He'd feel a deep sense of loneliness and confusion.