Well, start by having a clear idea of what you want to achieve with the fanfic. Are you aiming for a friendship - centered story or a more action - packed adventure? Then, develop the characters further. Maybe give Cameron a backstory that ties in with Ash's journey. For the plot, make it engaging. For example, they could be on a quest to find a legendary Pokemon that only appears once in a century. Include dialogue that is true to the characters. Ash would be enthusiastic and Cameron could be a bit more reserved at times. Also, make the settings come alive. Describe the Pokemon habitats and the towns they visit in detail.