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.
One possible plot twist could be that Tanjiro has to sacrifice a part of his own power to save Rengoku. Maybe he has to give up some of his newly acquired breathing techniques which he has worked so hard to master. This would show his selfless nature and his deep respect for Rengoku.
One common theme is adventure. Since both characters are from the Demon Slayer universe, they often go on demon - slaying adventures together in the fanfictions. They might face powerful demons and rely on each other's skills to survive.
You can try fanfiction platforms like Archive of Our Own (AO3). There are a large number of fan-created works there. Just use the search function and type in 'female Tanjiro x Rengoku' and you'll likely find a variety of stories with different themes and writing styles.
Well, one possible plot could be that Adrien is initially very conflicted about seeing his father in prison. He might be angry at his father for the things he's done that led to his incarceration, but also still have some lingering love or sense of family obligation. As he visits, his father could try to manipulate him into believing he's innocent or try to get Adrien to do something on the outside for him. Adrien would then have to struggle with whether to trust his father or not and figure out how this visit will impact his own life and his relationships with others.
Well, perhaps in the fanfiction, Adrien visits his father in prison to seek closure. His father could have committed some heinous crime that has affected many people, including Adrien. During the visit, Adrien might find out some secrets about his family that his father has been hiding, which adds more complexity to the story.