Some characters might be excited. They could be interested in seeing how their story continues in the fanfiction. Others might be skeptical and think that the fanfiction is not a true representation of their possible futures.
When characters watch the future D Gray - Man fanfiction, their reactions would depend on their personalities. The optimistic characters may look for the good in the fan - created futures and be inspired by them. However, the more pessimistic characters could focus on the negative aspects. For example, if the fanfiction shows a dark future for the Black Order, characters like Komui might be spurred into action to try and prevent it. Some characters might also have a hard time accepting the fanfiction's vision of their future if it goes against their own beliefs or desires. They may try to find flaws in the story or simply reject it outright.
The characters' reactions would vary greatly. For the more curious ones like Lavi, he might be really into it, trying to analyze every detail and see how it could relate to the real world he lives in. On the other hand, characters like Kanda might be dismissive at first. He might think it's just some silly made - up story. But if the fanfiction touches on something that he secretly fears or hopes for, he could start to take it more seriously. And then there are characters like Lenalee who might be a bit worried about the future presented, especially if it shows bad things happening to her friends or the world she is trying to protect.