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.
He might be shocked. In the fanfic, if he sees things like his friends' future achievements or the new threats that emerge, it would surely startle him. For example, if he discovers that one of his friends has a family of their own and has become a great leader in the village, he would be proud but also a bit surprised at how much has changed.
With his usual determination. Naruto has always been one to face difficulties head - on. So, even if he's shocked by what he sees in the fanfic's future, he'll likely try to find solutions immediately.
Naruto has been through a lot in his own time, so he may use that experience to adapt to the past. He might start by making friends with the younger versions of people he knows. He'll also have to be careful not to reveal too much about the future to avoid disrupting the timeline. He could use his ninja skills to earn respect and find his place in the past world.
Some Tumblr blogs are dedicated to sharing and promoting AOT fanfictions, including the reacts ones. You can search for relevant tags like #AOTreactsfanfiction on Tumblr. Also, Reddit has some subreddits dedicated to AOT. You might be able to find links to AOT reacts fanfictions in the posts or comments there.
One interesting story could be about the characters' reactions to different timelines. For example, Sans might react in a unique way when faced with a timeline where everyone is overly aggressive. Another could be the reactions of the monsters to human technology that they've never seen before, like a smartphone. And there could also be stories about how the characters react to fan - made scenarios, such as a 'what if' situation where Asriel didn't turn into Flowey.
Well, they might be really confused at first. The cultures and settings in Earth fanfictions are so different from their own world in RWBY. For example, if the fanfiction involves modern technology like cars or cell phones, which don't exist in their world, they'd be super curious and might have a hard time understanding how these things work. They could also be shocked by different social norms or historical events that are unique to Earth.
Some MHA reacts fanfiction focuses on the characters reacting to fan - made scenarios. For instance, they could react to a 'what if' situation like All Might never losing his power. This can show how the relationships and the overall story dynamics would change in such a scenario, and it's really fascinating to see the characters' responses.