A Madara's grandson fanfic might focus on his journey to understand his grandfather's actions. He could travel to the places Madara once frequented, meet people who knew him, and piece together a more complete picture. Maybe he'd find out that Madara had some noble intentions that were misconstrued over time. Along the way, he'd also develop his own skills and make friends and allies who support him in his quest for truth.
One possible fanfic could be where Madara's grandson discovers his family's legacy. He might struggle with the expectations that come with being related to such a powerful and controversial figure in the Naruto world. He could face both admiration and suspicion from others as he tries to make his own mark, perhaps by trying to bring a new era of peace or by exploring new jutsu that were inspired by Madara's techniques.