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.
If Luke were kidnapped, in the context of fanfiction, it might be the work of a new, unknown group. This group could be after the knowledge Luke has about the Force. His kidnapping could be a way to force him to reveal secrets. The Jedi Order would be in chaos, and his students, if he has any at that point, would be left without their master. They might try to find him on their own, leading to their own character development and adventures.
Yes, in some fanfictions, Tanjiro is depicted as the reincarnation of Yoriichi. This concept adds an interesting layer to the story as Yoriichi was an extremely powerful and legendary figure in the original lore. The idea allows for new storylines where Tanjiro might inherit some of Yoriichi's skills or have a deeper connection to the past events related to Yoriichi.
Tanjiro's fate in the manga is that he ultimately passed away. This event was a significant part of the story's development and had a profound impact on the other characters and the overall plot.
Tanjiro's strength in the manga is significant. He acquires new powers and techniques, and overcomes numerous powerful foes with determination and strategy.