It could have completely changed the dynamics of his battles and his relationships with others. The Sharingan would have given him an extra edge and new abilities.
If Naruto had the Sharingan in fanfic, he'd likely become even more powerful. His combat skills would improve as he could use the Sharingan to predict his opponents' moves more accurately. He might also be able to use some of the unique jutsus associated with the Sharingan. For example, he could potentially learn to use Amaterasu if the fanfic writer allows for that kind of power combination.
Tanjiro might use his sense of smell to detect hidden threats. Since his sense of smell is super - enhanced, he could sniff out any demons or traps in the way of saving Rengoku. For example, if there are demons disguised as allies, Tanjiro would be able to tell by their scent.
In a fanfic, Naruto with the Rinnegan and Sharingan would have a whole new set of challenges. He would have to deal with the jealousy and fear of other ninja villages. His own village might also be divided on how to handle his new powers. On the positive side, he could use these eyes to better understand the history of the ninja world. For example, the Rinnegan might give him insights into the Sage of Six Paths' true intentions, and the Sharingan could help him uncover secrets about his own family. He would also have to train hard to fully utilize the combined potential of these two powerful eyes, which could lead to some interesting training arcs in the fanfic.
One common theme is the exploration of gender roles. Since Tanjiro is male in the original, making him female in fanfic allows for a look at how she would be treated in the demon - slaying world as a female. Another theme is new relationships. For example, how she might interact differently with other characters like Zenitsu or Inosuke as a female.