Well, initially, there would likely be a lot of rumors and speculation. News would spread quickly throughout the Pokémon world. Some regions might celebrate it as a unique event, while others, especially those with more traditional views, might be more hesitant. The Gym Leaders might have different opinions - some could be open - minded and see it as a chance for new growth in the Pokémon world, while others might be worried about how it could change the established order of things. Overall, it would be a very eventful and controversial situation in the Pokémon world.
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.
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.
Hermione would probably be very shocked at first but then would immediately start looking for a magical solution. She might rummage through her books to find spells to reverse the situation or make it safe for Harry.
Ruby would be excited. Yang would find it cool. Weiss might be a bit skeptical at first, but Blake could see the potential for interesting stories.
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.
Well, perhaps the fanfiction could focus on how Rose's giving birth is related to a prophecy in the Pokémon world. Maybe the baby is destined to bring balance to a region that has been in chaos due to some powerful and evil Pokémon. The story could then revolve around protecting the baby from those who want to use it for their own evil purposes.
The Pokémon League might condemn Ash's actions. Despite Team Rocket's wrongdoings, killing is not an acceptable solution in the Pokémon world which values friendship and co - existence. Ash could face consequences such as being banned from Pokémon battles or certain areas. On the other hand, some trainers who have been harassed by Team Rocket for a long time might secretly support Ash.
The Pokémon community around them would probably be quite shocked at first. Their friends like Brock might be both surprised and happy for them. However, some of the more traditional Pokémon trainers might frown upon it as they are quite young. But Ash's close friends would most likely support them and help them out, like providing a place for May to rest during her pregnancy.
In fanfiction, the reactions in the Pokemon world to FLIM from MLP would be diverse. The Pokemon that are friendly and trusting, like Eevee, might initially be drawn to FLIM. However, as they get to know him better, they might realize his true nature. The villains in the Pokemon world, like Team Rocket, might see FLIM as either a competitor or a potential tool. They could try to use his skills for their own evil plans. Meanwhile, the regular citizens of the Pokemon world, such as the people in the various towns, would be cautious. They have a peaceful life, and FLIM's arrival with his business - minded and sometimes sneaky ways could disrupt their normal routine. So they would keep an eye on him to make sure he doesn't cause any problems.
The professors in the Pokémon world, like Professor Oak, would likely be concerned but also offer their wisdom. They could provide resources or knowledge about how to take care of a pregnant woman in the Pokémon world. The other trainers that Ash and Bonnie know might have a mix of reactions. Some might be supportive, while others might spread rumors or be jealous. Overall, the reactions would add a lot of drama and depth to the fanfiction.