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.