Superman would likely be impressed. He's used to being one of the strongest, but Goku's power levels would make him take notice. Batman would be his usual calculating self, studying Goku's moves for any potential threats or useful techniques. Flash would be excited to see someone who could keep up with his speed in a different way. Aquaman might be a bit more reserved, but still interested in this new kind of fighter from another universe.
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.
In a Goku Black - Justice League fanfiction, it's possible that Goku Black would initially be seen as a threat. But if there was some sort of deal or situation that made him side with the League, he could add a whole new level of power. His fighting style, which is very different from the traditional Justice League members, could make for some exciting battle scenes. He might even teach the League some new combat techniques.
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.
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.
In fanfiction, Goku Black could be incorporated into the Justice League as a sort of experiment. The League, always looking for ways to increase their power and protect the world, might see his abilities as a valuable asset. However, his different moral code and origin from another universe would create a lot of tension. There could be a story arc where the League tries to reform him or at least control his more violent tendencies while using his power to fight against threats that are beyond their normal capabilities.
Some members might be shocked and try to persuade him to stay. They know how valuable he is to the team, both in terms of his strategic mind and combat skills. Superman, for instance, might have a heart - to - heart talk with him, appealing to their long - standing friendship.
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.
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.