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.
Perhaps a powerful and corrupted Pokemon trainer could be the main villain. This trainer has a team of extremely strong and aggressive Pokemon. He sees Angie and Ash as threats to his goal of becoming the ultimate Pokemon master. So he constantly tries to undermine them, steal their Pokemon, or prevent them from achieving their own goals in the Pokemon world.
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 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.
It could be Team Rocket. They might have some sort of hold over Clemont, forcing him to betray Ash. Team Rocket has always been after Ash's Pikachu, and using Clemont to get to Ash would be a devious plan for them.
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.
It could be a powerful and evil Pokemon trainer who wants to capture Ash in his new Pokemon form. This trainer might be after Ash because he believes that a human - turned - Pokemon has some special power or value.
Maybe a powerful and evil pokemon trainer who has a vendetta against Ash. Now that Ash is in a vulnerable pokemon form, this trainer could try to capture and control him. The trainer might have some dark motives related to using Ash's unique situation to gain power in the pokemon world.
You can try websites like FanFiction.net. It has a large collection of Pokemon fanfictions, and you should be able to find some about Ash and Angie there. Just use the search bar and type in 'Pokemon Ash and Angie' and it will show you relevant stories.
One great Pokemon Ash and Angie fanfiction could be where they go on a journey to discover a new type of Pokemon. It might involve them facing challenges together and growing closer as friends. They could encounter rare Pokemon and use their combined skills to overcome difficulties.
A new villain, a mad scientist type, who is obsessed with the power of the Mega Buster. He wants to reverse - engineer it to create an army of robotic Pokémon with similar weapons.
In a new Ash Unova fanfiction, the main villain could be a scientist who is conducting unethical experiments on Unova Pokémon. He believes that by fusing different Pokémon together, he can create the ultimate Pokémon. His experiments are causing harm to the Pokémon involved and are also disrupting the natural balance in the Unova region. Ash discovers what's going on and has to stop the scientist, not only to protect the Pokémon but also to save the Unova region from the potential disasters that could be caused by these abnormal Pokémon creations.