The Hashiras would likely show concern. Some, like Shinobu, would immediately start looking into possible medical solutions. Giyuu might be more reserved in his worry but would still keep a close eye on Tanjiro. The other demon slayers would also be affected. They look up to Tanjiro, so his sickness would lower the morale of the group. They would try to support each other while waiting for Tanjiro to recover.
He might be really surprised. Naruto is used to his own world and the fanfiction could show things he never imagined. For example, if the fanfiction has his kids achieving great things in new ways, he'd be proud yet a bit shocked at how creative fans can be.
He might be shocked at how the village has developed. In some fanfictions, the village could have advanced technology. Naruto, being from a more traditional ninja world, would likely be both amazed and a bit confused at first.
He might be shocked at first. In fanfiction, the future could be full of unexpected changes. He could be excited to see how his friends and the village have developed. Maybe he'd be a bit sad if things didn't turn out as he hoped for his loved ones.
Well, they may react with a bit of nervousness too. The future in the fanfic could bring new challenges like changes in the music industry trends. For example, new technologies might require them to adapt their performance and promotion methods. They might worry about whether they can keep up with the fast - paced changes while still staying true to their art. But at the same time, they would also be determined to face these challenges head - on because they have always been a group that thrives on growth and evolution.
If the future she visits is a bad one, Hermione would be determined to change it. She has a strong sense of justice and wouldn't stand by and let people suffer. She would use her knowledge of the past and her wits to find a way to make things better, whether it involves changing the actions of certain individuals or influencing the course of events on a larger scale.