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.
Perhaps in the fanfiction, Harry's connection to the Dark Mark leads him to discover a hidden side of Voldemort's past. This could involve uncovering secrets about Voldemort's family or the origin of the Dark Mark itself. He could then use this knowledge to try and find a way to break the connection or use it against the Dark forces in some way. For example, if he finds out that there's a certain spell that was used to create the Dark Mark and that same spell can be reversed, he might embark on a journey to find the ingredients or the incantation needed for that. And all the while, he has to deal with the prejudice and fear from those around him who might suspect him of being a Death Eater.
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.