It could be a Pokemon itself. Maybe a dark - type Pokemon with a grudge against humans and Pikachu in particular. This Pokemon could be using its powers to cause chaos in the city, and Pikachu has to figure out its motives and stop it before it's too late.
It could be a mad scientist. The scientist has been researching ways to combine organic and electrical energy. He sees Pikachu as the perfect specimen and Samus as someone who will try to stop his unethical experiments. So he sets out to capture Pikachu and eliminate Samus.
Detective Pikachu is surely one of the main characters. There might also be other Pokemon and human characters that play important roles, like Tim Goodman who is often associated with Pikachu in the Detective Pikachu storylines.
It could be a new character, a powerful dark wizard who wants to use the combined power of Naruto's chakra and Pikachu's electricity for his own evil purposes. He creates all sorts of dark creatures and sends them after Naruto and Pikachu to try and capture them. He's very cunning and has a lot of minions to do his bidding.
It could be a trainer who hates Pikachu and wants to capture all of them. This would put Ash, as a Pikachu fan, in direct conflict with this trainer.
A possible main villain could be a character from Ladybug's past who has discovered her identity and is using that knowledge to create chaos. They could be blackmailing her or trying to steal her Miraculous by setting up elaborate mysteries and traps for her to solve.
One idea could be to create an OC who is a detective from a different region, like Unova. They could have a unique Pokemon partner, like a Zorua that can transform and help with investigations in sneaky ways. Their backstory could involve coming to Ryme City to learn new detective techniques.
Another possibility is a rival trainer who has a Pokemon that is weak against Raichu. This trainer might try to prevent Pikachu from evolving to keep their own Pokemon's advantage in battles. They could try to steal Pikachu's food or training resources to slow down its growth and evolution.
Perhaps an evil Pokemon scientist who has created a device that can control the minds of Pokemon. He discovers Naruto's arrival and sees him as a threat. So, he teams up with some of the most powerful and aggressive Pokemon to stop Naruto. He has no regard for the well - being of Pokemon or the balance of the world, only his own ambition to rule.
A new villain could be a powerful and dark psychic - type Pokemon. This Pokemon could have the ability to turn Ash into a Pokemon as part of its grand plan to take over the Pokemon world. It sees Ash, who has always been a symbol of hope and a strong trainer, as a threat even in his human form. So, by turning him into a Pokemon, it hopes to disrupt the balance and order that Ash has been a part of maintaining.
A group of Team Rocket - like villains could be the main antagonists. They might want to capture Pikachu while it's in the process of evolving because they believe that an evolving Pikachu would have some special power that they could use for their evil plans. They constantly try to disrupt Pikachu's evolution and the bond between Pikachu and its trainer.