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.
It could be that Aggron gets into some trouble, like being captured by an evil team. Ash then has to come up with a plan to rescue it. He might use his other Pokemon and his wits to outsmart the bad guys. Along the way, we could see how much Ash cares for Aggron and how far he'll go to save it.
One idea could be that Ash discovers a Sylveon in a forest. The Sylveon is injured and Ash takes it upon himself to nurse it back to health. This could lead to a strong bond between them, and Sylveon could become a unique addition to his Pokémon team, with new adventures and battles ensuing.
If we consider the common growth pattern in the Pokemon series, Charmander evolving into Charmeleon could be an early evolution in the fanfiction. Since Charmander has a clear evolution line and Ash has a strong bond with it, it would make sense for it to be one of the first to evolve, perhaps due to a tough battle or a special encounter that accelerates its growth.
Some Pokémon - related forums also might have 'aggron ash fanfic'. These forums are often a great place for fans to share their creative works, so it's worth checking them out.
One great 'aggron ash fanfic' could be one where Ash and his Aggron go on an adventure to explore a mysterious new region. They might encounter rare Pokémon and face challenges together.
Obviously, Ash is one of the main characters. He's the protagonist who has a passion for pokemon and is always eager to become a pokemon master. And the legendary pokemon he encounters or befriends would also be important characters. For instance, if it's Lugia, Lugia would play a major role in the story as it's a very powerful and mysterious pokemon.
His Pokemon might be a bit confused at first. Pikachu, for example, is used to their adventures on the road. But they'll soon adapt. They could make new Pokemon friends at the school too. And they'll be there to support Ash during his school challenges, like in battles or tests.
In fanfiction, Ash might improve his pokemon's IV through special training regimens. For example, he could focus on specific battle scenarios that target areas related to IV stats like speed or strength. He might also seek out rare items in the fanfiction world that have the power to enhance these inherent values.
I'm not entirely sure as I haven't read that specific fanfiction. But generally, it might be about Ash's adventures with his Pokémon in a rather unique or different way compared to the original Pokémon story. It could involve new challenges, relationships, or character developments for Ash and his Pokémon.