It's hard to say exactly who the main characters would be without reading the specific 'pokemon titan fanfic'. But it could be a young and brave trainer who is determined to face these titan Pokemon. Maybe there's also a wise old Pokemon professor who provides guidance on dealing with them.
A 'pokemon titan fanfic' could be about many things. It might focus on powerful, titan - like Pokemon in the Pokemon universe. It could be a story where trainers encounter these gigantic and perhaps more dangerous versions of regular Pokemon, and the adventures and challenges that follow.
The world of Attack on Titan would experience a major transformation with the introduction of Pokemon. Socially, the relationship between people would change as they bond over their Pokemon. There could be Pokemon centers and Pokemarts within the walls. Economically, a new industry would emerge around Pokemon. In terms of combat, as I mentioned before, different types of Pokemon would bring new tactics. Fire - type Pokemon could set the Titans on fire, ice - type Pokemon could freeze them, and so on. Also, the idea of Pokemon evolution could be incorporated into the story. Maybe the more a trainer and their Pokemon fight Titans together, the stronger their bond and the more evolved their Pokemon becomes, making them even more effective against the Titans.
The character interactions can be really fascinating. Imagine Mikasa Ackerman teaming up with a powerful Pokemon like Charizard. She could ride on its back and use her combat skills along with Charizard's fire - breathing abilities to take on Titans. Also, the idea of the Pokemon having to adapt to the world of Attack on Titan, where there are different rules and a lot more danger, is an aspect that can be explored in many interesting ways.
One popular 'Attack on Titan Pokemon Crossover Fanfic' might be 'The Titans and the Pokeballs'. It combines the intense world of Attack on Titan with the unique creatures of Pokemon. In this fic, characters from Attack on Titan might find themselves in a situation where they have to use Pokemon to fight the Titans. Another could be 'Pokemon in the Walls', which could explore how Pokemon would fare in the world of Attack on Titan, with trainers using their Pokemon skills to survive the Titan attacks.
Eren could be an interesting choice. Since he has such a complex character arc already, making him the tenth titan could add another layer to his story. He could struggle with the new power and how it changes his view of the world and his mission.
One possible fanfic could be about Mikasa's transformation into a Titan being a unique situation in the 'Attack on Titan' world. Maybe she has different abilities compared to other Titans. For example, she could retain more of her human consciousness and use her combat skills in Titan form. This would create an interesting dynamic as she tries to protect her friends while being a Titan, which is usually seen as an enemy.
One possible fanfic plot could be that Eren has been planning this for a long time. He might gather intelligence about the Colossal Titan's weaknesses. Then, in a fierce battle, he manages to get close enough to deliver a fatal blow. This could be a very emotional moment for him as well, considering the history between them.
A great plot for such a fanfic could be that Mikasa's Titan form is a result of a connection to an ancient Titan power. As she becomes a Titan, she starts to have visions of the past, of how the Titans came to be. She then decides to use this knowledge and her new form to try and end the Titan - human conflict once and for all. Along the way, she has to face her friends who are initially scared of her but eventually come to accept her new form as she proves that she is still the same Mikasa deep down, just in a different body.
Yes, in some Percy Jackson fanfics, he could be depicted as a Titan. Fanfic writers often take creative liberties and might come up with unique storylines where Percy has Titan - like qualities or is even reimagined as a Titan for the sake of an interesting plot twist.