They are likely to be horrified. James, for example, would probably be extremely angry that his friend had to endure such a thing. He might immediately vow to protect Sirius from any further harm. Remus, being more empathetic, would be there to comfort Sirius and offer his support in a more quiet and understanding way. Peter, though not as strong - willed as the others, would still be shocked and follow the lead of James and Remus in trying to make Sirius feel better.
The marauders' reaction would be quite complex. They might feel a sense of nostalgia as they read about the world they once knew, but through a different lens in the fanfiction. They could also be a bit critical. If the fanfiction didn't seem to respect the essence of the original story, they might be quick to point it out. However, if it was well - written and added new depth to the story, they could end up really enjoying it and even discussing it at length among themselves.
The marauders would probably have to adapt their way of thinking. In the future, the concepts of right and wrong, good and bad might have shifted. They'd need to understand these new perspectives to fit in.
Remus might be a bit more reserved. He'd approach the fanfics with caution, but he'd also be touched by the stories that explored his struggle with being a werewolf in more detail. He might find comfort in knowing that fans understood his inner turmoil to some extent.
They might be very amused. The marauders are known for their sense of humor and the 'A Very Potter Musical' fanfiction is often full of comical and over - the - top elements. So they'd probably have a good laugh at the silly situations and the creative liberties the fanfic takes.
In this fanfiction, it could be that the future has a new Dark Lord. The Marauders are brought forward in time to stop this new threat. They face challenges like understanding the new forms of magic and technology that the future Dark Lord has harnessed. They also have to deal with the fact that the future wizarding world has different social and political structures. They might start by infiltrating the Dark Lord's lair using their Marauder's Map - like skills, but find that the map doesn't work as well in the future, leading to all sorts of interesting and dangerous situations.
They might be shocked by the advanced technology. For example, seeing flying cars and instant communication devices everywhere could leave them in awe.
One possible plot could be that the Marauders use a time - turning device by accident and end up in the future. They might be shocked by the advanced technology and different social norms. They could also meet their descendants or versions of themselves that are very different from what they expected.
Perhaps the Marauders watch the future and are shocked to find out that some of their group won't survive the upcoming wizarding wars. They might then start to take extra precautions and train harder to change that future. They could also see new friendships and alliances being formed in the future that they then try to encourage in their own time.
They would probably be very curious at first. But as they read more, they might get annoyed if the fanfiction doesn't stay true to their essence. For instance, if a fanfiction portrays Sirius as a coward, which is completely against his real character, they would be offended. However, if they find a well - written fanfiction that respects their characters, they might enjoy it.