It's hard to say for sure. A lot of factors come into play, like the complexity of the story, the level of detail, and how well it keeps the reader's interest. So, it varies from person to person.
The protagonist of Pokemon Yellow is an obvious potential ally. They are on their own journey to become a Pokemon master and could easily team up with Lance. Another could be Professor Oak. He has a vast knowledge of Pokemon and could provide valuable information and resources to Lance. Also, the Gym Leaders in the Pokemon Yellow world could be allies. They are all skilled trainers in their own right and could join forces with Lance when facing a common threat, like an evil team trying to disrupt the Pokemon League.
One idea could be a story where the main character discovers a new, rare Pokemon in an unexpected place, like in a hidden cave in Viridian Forest. Another could be a fanfiction where the Pokemon trainers from Red and Yellow team up to face a new, powerful villain who has a unique ability to control Pokemon in a different way than the usual villains in the games. Maybe also a story about a trainer's Pokemon evolving in a very different way due to some strange phenomenon in the Kanto region.
One idea could be to focus on a new trainer's journey in the world of Pokemon Yellow. They could start with a Pikachu that has a unique ability not found in the original game.
Another popular character is Pikachu. Since it's the iconic Pokemon in the Yellow version, fanfictions often center around Pikachu's unique bond with its trainer. It could be a story about Pikachu's evolution journey, or how it saves the day in a tough situation.
Red is definitely a popular character to feature. He's the main protagonist in the games, so many fanfictions revolve around his further adventures or his backstory. Another popular one is Pikachu, the iconic Pokemon. You can write all sorts of stories about Pikachu's relationship with Red or other trainers. And of course, Blue is often in fanfictions as well, as he's Red's rival and their dynamic can be explored in many ways.
A cool plot could be that Lance, in the Pokemon Yellow world, decides to start a new type of Pokemon training school. He recruits the protagonist from the game as his first student. Together, they develop new training techniques and discover new Pokemon evolutions. But an old rival of Lance hears about this and tries to sabotage their efforts. So they have to not only deal with the challenges of training and discovery but also fend off this rival who is jealous of Lance's new project.
First, familiarize yourself with the game's mechanics and story background. Then, keep an eye out for special interactions and dialogues that might provide hints for the walkthrough. Sometimes, collaborating with other players or referring to online forums can also be useful.
It involves uncovering the hidden clues and solving puzzles to progress through the story. You need to pay attention to details and character interactions.