Well, they might be shocked at how much technology has advanced in the future depicted in the fanfiction. Maybe there are new types of elemental powers or different ways of training ninjas. It could also be that the relationships between characters have changed in unexpected ways, and they would have to come to terms with that.
They might gain a new perspective on their own abilities. If the future fanfiction shows ninjas with enhanced powers, the past characters could start to think about how they could develop their skills further. For example, if in the future they can combine elemental powers in new ways, the past ninjas might start experimenting.
I'm not really sure what you mean by 'kanao comics hentai'. It might refer to some adult-oriented or inappropriate content related to a particular character.
In the manga, Tanjiro and Kanao don't end up getting married. The story focuses more on their battles and character development rather than romantic pairings.
Tanjiro and Kanao's love is a slow - burning and sweet one. They first meet as comrades in the fight against demons. Tanjiro's kind heart and strong will gradually draw Kanao out of her shell. Kanao, who was initially rather reserved, starts to open up more because of Tanjiro. Their relationship develops as they face numerous challenges together, and their mutual respect and growing affection for each other are heartwarming to watch in the series.
It develops gradually. At first, they are just fellow Demon Slayers. But as they spend more time together, they start to understand each other better. Tanjiro's unwavering determination and his ability to see the good in everyone, including Kanao, make her fall for him. Kanao also shows her loyalty and care towards Tanjiro, which deepens their connection.
One great tanjiro and kanao fanfiction could be where they go on a journey together to find a rare herb for an ailing comrade. In this story, their relationship deepens as they face various obstacles in the forest.