The Flash would likely find it really entertaining. He might enjoy reading about his super - speed being used in all sorts of crazy ways in the fanfiction. And he could also use it as a way to pass the time when he's not out saving the day. He might even start to wonder if some of the ideas in the fanfiction could actually work in real - life superhero situations.
The Justice League as a whole would have a diverse range of reactions. Some members like the Flash would be all for it, seeing it as a fun exploration of Batman. Others like Wonder Woman might use it as a way to understand how the public perceives Batman's role in the League. And then there are those like Aquaman who might just shrug it off unless it directly affects his interests. The fanfiction could also potentially create some internal discussions within the League about how they are all portrayed in the public's imagination.
Some of them might laugh. They would see the creative and sometimes out - there ideas that fans come up with in the fanfiction. For example, if there's a fanfic where Oliver Queen teams up with a really strange character from the Arrowverse, it could be quite humorous to them.
The Flash might initially be in denial. He's used to Batman always having a plan and his death would be a huge shock. He would then turn his speed into a tool for investigation, trying to find out exactly what happened. Green Lantern could feel a sense of loss but also be spurred into action, using his ring to try and prevent future tragedies like Batman's death. He might create constructs in Batman's memory to inspire the team.
Some of them might be amused. They could find the different takes on their characters and stories quite entertaining. For example, Superman might chuckle at a fanfic that has him in a comical situation.
Superman might be a bit amused. He'd see the Naruto characters' struggles and compare them to the challenges he faces in Metropolis. Batman would probably study it like he does any case, trying to figure out the motives of the characters in the fanfiction. Wonder Woman would likely be drawn to the strong female characters in Naruto fanfiction, if there are any.
They would be amazed. The Justice League is used to their own brand of super - powered battles, but Infinity War would show them a whole new level of cosmic warfare. Superman might be the first to jump into action, mentally preparing himself to take on Thanos if he was in that universe.
They'd likely be intrigued. Batman would study it for plot devices and character development. He might use some of the ideas in his own crime - fighting strategies. Superman would probably be entertained, seeing it as a new form of storytelling about a hero. Wonder Woman might view it as a cultural phenomenon and think about how it reflects on modern society's view of heroes.
Batman would probably be more critical. He'd analyze how the fanfiction portrays his detective skills and his relationship with the other members of the Justice League. He might not like it if the stories make him seem too soft or too brutal, as he has a very specific self - image.
Some might be amused, while others could be offended. It depends on how they are depicted in the fanfiction.
He'd probably be confused at first. Naruto is used to being among friends and comrades. But then, his determination would kick in. He'd likely try to talk to them, to understand why they think he's a threat.