It could be a Team Plasma - like organization that is causing trouble in Unova. They might be trying to steal Pokémon or disrupt the balance in the region, and Ash and May have to stop them.
Perhaps there is a rogue trainer in the fanfiction who is really strong and has a grudge against Ash and May. This trainer always tries to sabotage their journey and challenges them to unfair battles. He or she might have some dark secrets or motives related to their own past in the Unova region.
There could be a new, unknown group of villains in the fanfiction. They might be after a special type of Pokemon that is native to Unova. These villains could be using underhanded tactics to catch these Pokemon, and Ash and May would have to uncover their motives and stop them. For instance, they could be trying to use the power of these Pokemon for their own selfish gains, like taking over Unova's Pokemon centers or disrupting the Pokemon league.
A powerful and greedy Pokémon collector could be the main antagonist. He wants to capture all the rare Pokémon in Unova for his own collection, regardless of the consequences for the Pokémon or the ecosystem. He sees Ash and May as obstacles in his path, especially when they try to protect the Pokémon they encounter.
In such a fanfiction, Ash and May could encounter new and rare Pokemon in Unova. They might face tough battles against powerful trainers and gym leaders. For example, Ash could use his Pikachu in unique strategies against Unova's electric - type Pokemon that have different abilities compared to those in other regions.
Well, sometimes the antagonist can be an internal struggle within Ash or May themselves. For example, Ash's over - ambition to become the best Pokémon trainer might lead him to make choices that seem like a betrayal to May. His single - minded focus on winning battles and collecting badges could make him neglect May's feelings. In this case, Ash's own ambition is acting as the antagonist in the story. Similarly, May's insecurities could make her misinterpret Ash's actions as betrayal, and her insecurities then become the antagonist in the relationship.
Team Rocket could be the main antagonists. They're always causing trouble for Ash and his friends, and they'd surely be after Ash and Leaf on their journey too, trying to steal their Pokémon.
One interesting plot point could be Ash and May encountering a rare and powerful Unova Pokémon that they have to work together to catch. This could test their teamwork skills and knowledge of Pokémon.
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.
Some potential rivals could be the local Unova trainers who are known for their unique battling styles. For example, a trainer who specializes in using Bug - type Pokemon could be a tough rival for them.
Obviously, Ash is one of the main characters. Since it's about 'smart Ash', his intelligence will likely play a big role in the story.
One possible plot could be that Ash and Dawn explore Unova together, facing new Pokemon and trainers. They might encounter a rare Pokemon that is causing trouble in the region and work together to solve the problem.