Perhaps a powerful and mysterious trainer. This trainer is jealous of Ash's connection with his Pokemon and his ability to conduct these experiments. He wants to prove that he is the better trainer. So he tries to disrupt Ash's experiments by challenging him to battles at inconvenient times. He also spreads false rumors about Ash's experiments being dangerous to other Pokemon, which causes some local Pokemon communities to be hostile towards Ash.
One possible main antagonist could be a powerful and arrogant trainer who wants to prove that he is better than Ash and Nora. He might try to steal their Pokemon or sabotage their battles.
Perhaps a group of bandits who steal Pokemon and sell them on the black market. They see Ash and Serena as threats because they are trying to stop their illegal activities. These bandits could be led by a really tough and mean - looking guy who has no mercy when it comes to getting what he wants. They would constantly be trying to sabotage Ash and Serena's attempts to protect the Pokemon and the people in the medieval setting.
It could be Team Rocket. They are always causing trouble for Ash and his Pokemon in the Pokemon series, so it's likely they would be the antagonists in this fanfiction too.
Often, Team Rocket can still be a main antagonist. They might come up with new and devious plans to steal Ash's Pikachu or Serena's Pokemon. Their comical yet persistent attempts to cause trouble add a lot of action and humor to the fanfiction.
Well, it could be about Ash experimenting with new battle strategies. For example, he might try to combine different Pokemon moves in unexpected ways. Maybe he'd experiment with training his Pokemon in unique environments like a mysterious forest filled with rare Pokemon. This could lead to his Pokemon evolving in new and exciting ways, and he could discover new Pokemon abilities along the way.
One interesting story could be about Ash being part of a secret Pokemon evolution experiment. In this fic, scientists might be trying to force-evolve Pikachu in a way that goes against nature. Ash has to fight to protect Pikachu and the bond they share. It shows his determination to keep his Pokemon's well - being a top priority.
There could be a rogue legendary Pokemon that sees Ash's transformation as a threat to the natural order. This legendary Pokemon, with its own set of motives, could be trying to prevent Ash from becoming a fully - fledged legendary - like being. It might set up various obstacles and battles for Ash, testing his will and strength at every turn.
A group of rogue scientists who are experimenting on Pokemon in unethical ways could be new antagonists. They might be creating powerful and dangerous hybrid Pokemon.
There could be a new group of villains. Maybe they are a secret organization that has been watching Ash for a long time. They fear that his secret could disrupt their plans for world domination in the Pokemon world. So, they constantly try to stop him from uncovering his own secret.
You can try websites like FanFiction.net. It has a large collection of Pokemon fanfictions, and you can search specifically for ones related to Ash and Pokemon experiments. Just use the search bar and type in relevant keywords like 'pokemon experiment ash' and you'll likely find a bunch of stories.