Team Rocket would be a likely main villain. They are a big part of the Pokemon Red and Blue story, and when Ash enters this world, they would surely cause trouble for him. Their schemes to steal Pokemon and take over the world would be a direct threat to Ash and his new friends.
Another possibility is that a corrupted Pokemon could be the main villain. In the world of Pokemon Red and Blue meets Ash, a Pokemon might be influenced by some dark power. This Pokemon could start causing chaos, and Ash would have to figure out how to stop it, perhaps by finding a way to purify it or defeat it in battle.
It could also be a new villain created specifically for this fanfiction. Maybe a powerful trainer who has a grudge against both Ash and the main characters from the games. This trainer could have some rare and powerful Pokemon that Ash and the others have to face in order to protect the Kanto region.
In a 'pokemon red and blue meets ash' fanfiction, Ash could face some challenges related to the game's story. For example, he might get involved in the conflict between Team Rocket and the Pokemon League. His Pikachu would surely be a key player in battles against the strong Pokemon in the game world. He might also discover new things about Pokemon evolution and training methods that are different from what he already knows.
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.
The main characters would surely include Ash. In Pokemon Red, the rival character, Gary (or whatever name you choose for the rival in the game), could also play a major role. And of course, the various gym leaders from Pokemon Red. They could have different encounters with Ash, some might be friendly and help him, while others could be more challenging opponents. Also, Professor Oak might be involved, as he is connected to both Ash's world and the world of Pokemon Red in some ways.
In the 'pokemon red meets ash fanfiction', Ash could potentially encounter the original starter Pokemon from Pokemon Red in a new and exciting way. He might have to face different challenges related to the game's mechanics, like dealing with the different gym leaders' strategies that are specific to Pokemon Red. And it's possible that he forms new friendships with the game - exclusive characters.
In this fanfiction, Ash might learn some new battle strategies from Red. Since Red is a very experienced trainer, he could show Ash some unique ways to train Pokémon.
Obviously, Ash and Red are the main characters. Ash is the well - known protagonist from the Pokemon anime series, and Red is a famous character from the Pokemon games.
A new evil organization could be created for the fanfiction. They could be a group of scientists who want to use Pokemon for their own cruel experiments. Angie and Ash find out about their plans and have to stop them before they harm too many Pokemon. They might be led by a mysterious figure who has a personal vendetta against Ash for some reason.
A powerful and greedy Pokémon collector could be the main villain. He wants to capture all the rarest Pokémon for his own collection, regardless of the harm it does to the Pokémon and their habitats. Ash and Caroline would have to stop him from achieving his evil goals.
Dr. Eggman could team up with a Pokemon villain like Team Rocket. They could combine their technology and evil plans. Eggman might use Team Rocket's knowledge of Pokemon to capture strong Pokemon and use them in his machines, while Team Rocket could benefit from Eggman's advanced robotics to create more powerful devices for stealing Pokemon.
Maybe they'll go on an adventure together in Alola. Red could be amazed by the unique Pokémon species in Alola, and Ash could learn from Red's experiences with Pokémon from other regions. They might also team up to face some tough opponents and show off their combined skills in Pokémon battles.