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 many Justice League fanfictions where Batman dies, it could lead to a complete shift in the team dynamics. For example, the League might become more reckless without Batman's strategic mind. Superman could struggle with the loss, perhaps blaming himself for not being able to protect Batman. The other members like Wonder Woman would have to step up in terms of leadership and planning. It could also open the door for new characters to emerge in the fanfiction's storyline, trying to fill the void left by Batman.
One possible plot could be that Naruto somehow gets transported to the DC universe where the Ace Justice League exists. He could initially be seen as a threat due to his strange powers. But as they get to know him, they realize his good intentions. Naruto could teach the League some of his ninja techniques, like how to use chakra to enhance their physical abilities. And in return, the League could introduce Naruto to advanced technology and different ways of justice in their world.
Well, in a fanfiction, anything can be possible. Naruto's skills like his shadow clones and Rasengan could potentially be used to rescue Ace. Maybe he uses his speed to get through the Justice League's defenses, which are set up around Ace for some reason. And then he uses his clones to create a distraction while he frees Ace with his combat abilities.
The Justice League's reaction in fanfiction when Batman dies can vary greatly. Superman might become more withdrawn, feeling like he failed as a friend and protector. Wonder Woman could be the one to try and hold the team together, using her strength and wisdom. The Flash might struggle with guilt, thinking he could have been faster to save Batman. Each member would have their own internal conflict and way of dealing with the loss, which could lead to some interesting character development in the fanfiction.
In some fanfictions, Batman might sacrifice himself to save the Justice League from an all - powerful enemy. For example, facing a threat that can only be stopped by using a dangerous self - destruct device that Batman activates while the others escape.
Well, if Naruto dies in a Justice League fanfiction, it might start with Naruto being transported to the DC universe for some unknown reason. There, he might get involved in a fight that's too big for him to handle, resulting in his death. This could shock the Justice League members as they are not used to this type of ninja - like power. Batman would probably start investigating Naruto's origin and his death. Aquaman might be concerned about any potential effects on the oceans if Naruto's powers had something to do with water. Flash could be interested in how Naruto moved so fast. His death would be a major event that would disrupt the normal flow of the DC world.
The plot could start with Ace being in trouble in the Justice League's world. Maybe he's been captured by an enemy of the League. Naruto somehow gets wind of this and decides to help. He travels to the Justice League's realm and starts his search for Ace. Along the way, he meets some of the Justice League members who are initially skeptical of him but then see his potential. Together, they find Ace and Naruto uses his jutsu to defeat the bad guys and free Ace.
Perhaps a member of the Justice League, accidentally. For instance, during a fight where powers go out of control. Let's say Flash runs too fast and accidentally knocks Batman into a sharp object, causing a fatal injury. This would create a huge moral dilemma within the League.
Another possibility is Flash and Naruto. Flash, with his super - speed, would be one of the first to reach Naruto if he suddenly appears in the DC world. Flash's friendly and curious nature would lead him to start a conversation with Naruto right away. Naruto would be amazed by Flash's speed and might try to compete with him in some way, which could be the start of their interaction.