Hiruzen Sarutobi is definitely one of the most important. As the Third Hokage, he has a wealth of knowledge and power. His actions and decisions have a huge impact on the clan and the village. Another important character could be Asuma Sarutobi. He is a skilled ninja and his relationships with other characters, like Shikamaru, add depth to the fanfiction.
One idea could be exploring the early days of the Sarutobi clan. Maybe show how they developed their unique jutsu. Another could be a story centered around a young Sarutobi clan member who has a rare ability and must prove themselves in the Hidden Leaf Village.
One possible plotline could be Naruto growing up with a different set of values and training methods. Instead of the rough start he had in the original story, he'd be mentored by the Sarutobi clan's elders. He might learn more about the history and secrets of Konoha at an earlier age, which could lead to him being more politically aware and better equipped to handle future threats. For example, he could be trained in unique Sarutobi jutsu from a young age, giving him an edge in combat.
His character would likely be more refined. With the Sarutobi clan's influence, he'd have better access to knowledge and proper guidance. He might not be as brash as in the original story. For example, he'd learn the importance of diplomacy and strategy earlier.
One of the popular characters paired with Sarutobi is Jiraiya. They are both important figures in the Hidden Leaf and their relationship, whether it's as comrades or as teacher - student in some fanfic interpretations, is always interesting to explore.
Obviously Naruto would be one of the main characters interacting with him. Naruto would look up to Sarutobi as a mentor and a source of knowledge. Their interactions could range from training sessions to deep conversations about the future of the village.
Perhaps he uses some ninja techniques like a shadow clone jutsu. He could make multiple clones to help him sort through and complete the paperwork quickly.
One characteristic could be the exploration of relationships. Since it's a fanfic, it might delve into relationships that Sarutobi has with other characters in the original work or even create new relationships. The 'lemon' aspect, if not referring to something inappropriate, could imply a certain zesty or interesting element in the story, like a plot twist or a unique interaction that adds flavor to the overall narrative.
In this fanfiction scenario, Naruto might learn unique chakra control techniques from Sarutobi. Sarutobi, being an experienced ninja, could teach Naruto how to better manage his large chakra reserves. This would lead to Naruto being able to perform jutsus more efficiently and with less waste of chakra.