First, re - watch the movie or read about the real - life events related to it. This will help you get a better understanding of the characters and the setting. Then, think about a unique angle or idea you want to explore, like a different approach to solving the enigma code. After that, start with a simple outline of the plot, including the beginning, middle, and end. And don't be afraid to let your creativity flow.
Begin by researching the historical context of the Imitation Game. This will give your fanfiction authenticity. Then, consider the relationships between the characters and what could be changed or expanded upon. For example, you could explore a romantic relationship between two characters that was only hinted at in the movie. Next, create a conflict or a problem that the characters need to solve. It could be something related to the code - breaking or something more personal. Write down your ideas as they come to you, and then start structuring them into a coherent story.