The main characters could be the Fire and Ice Pokemon themselves. For example, a Flareon and a Glaceon. They could be on a quest to find a special crystal that can balance the power of fire and ice in their world. Their interactions and growth throughout the journey would be the focus of the fanfic.
The main characters could be a young, inexperienced trainer who has just caught their first Fire - type and Ice - type Pokemon. As they journey through the Pokemon regions, they learn about the strengths and weaknesses of their Pokemon, and grow as a trainer. Along the way, they meet other trainers and Pokemon that help or hinder their progress. For example, an older, more experienced trainer who tries to convince the young trainer to give up their Ice - type Pokemon because it's 'too weak' in their opinion.
Well, there could be the dragon lords. One might be a powerful fire dragon lord who is fierce and protective of his kind. And on the ice side, a wise ice dragon lord who is known for her strategic thinking.
Arya Stark from 'Fire and Ice' could be a great main character in a 'Fire and Ice Twilight Fanfic'. She has the skills of a trained assassin and a strong will. In the fanfic, she could find herself in the world of 'Twilight' and use her skills to navigate the dangerous world of vampires and werewolves. Maybe she would form an alliance with the Cullens or the werewolf pack and help them in their battles.
Since I don't have detailed knowledge of this 'Fire and Ice Novel', the main characters could be individuals with a connection to the elements of fire and ice. Maybe there is a character who has the power to control fire and is in conflict with another who can manipulate ice. There could also be a character who is trying to bring peace between the two factions associated with fire and ice.
Naruto himself could be a main character. He has a natural inclination towards befriending powerful beings, and his determination would make him a great fit to interact with the fire and ice dragons. Sasuke could also be a main character, with his Sharingan potentially having some unique interactions with the dragons' powers.
A determined young trainer could be the main character. They have a special bond with Pokémon and are drawn to the mystery of the Sacred Fire. Their goal could be to learn more about it and protect it if necessary.
Hot Pie is a character that could be really interesting. He's just a simple baker's boy, but he has his own dreams and fears. You could write about his adventures after leaving Arya, how he tries to make a name for himself with his baking skills in different parts of Westeros. Another one is Podrick Payne. He has shown loyalty and courage, and a fanfic could explore his growth from a young squire to a more confident knight - like figure.
Hot Pie. He's just a simple baker boy, but he could have an interesting story. Maybe he uses his baking skills to gain influence in a small town. He could create special pastries for lords and ladies, and in the process, get involved in local politics or even some minor intrigues.
I'm not sure as the specific 'fire and ice short story' isn't fully defined here. But usually in such a story, there could be a character who is associated with the element of fire, perhaps a passionate and impulsive person. And another character might be linked to ice, a cold and calculating one.
I'm not sure specifically as it's a 'new' novel. But usually in a fire and ice themed novel, there might be a fearless warrior as one of the main characters.