One interesting mha reacts fanfic could be where the characters react to seeing their future selves. It adds an element of mystery and self - discovery. For example, Midoriya seeing himself as a fully - fledged hero might make him realize how far he has come and what he still needs to achieve.
First, know the characters well. Understand their personalities, motives, and backstories. This will make their reactions more believable. For example, if you're writing about Todoroki reacting to something, you need to consider his complex family situation and how it shapes his views.
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.
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.