Green Arrow, if he were part of the reactions (even though he's not always a core Justice League member), would probably make some sarcastic comments. He might joke about how the fanfiction writers are making Batman seem more of a mystery than he already is. Martian Manhunter, with his ability to read minds, could be interested in why the writers created certain scenarios for Batman in the fanfiction.
The reactions of the Justice League would also depend on the genre of the fanfiction. If it's a romantic fanfiction involving Batman, some members might be really shocked or amused. For example, Wonder Woman might raise an eyebrow at the idea of Batman in a romantic relationship in the fanfiction. If it's a dark and gritty fanfiction, the more serious members like Batman himself might appreciate the darker take on his character, while others might find it a bit too extreme.
Superman could be quite curious. He might read it to see how people view Batman from an outside perspective. Maybe he'll find some new insights into his friend. Batman himself might be a bit embarrassed or annoyed, depending on how the fanfiction is written. If it's full of inaccuracies, he'll likely not be happy.
Another storyline might involve Batman leaving because of a personal crisis. For example, he discovers something dark about his past that makes him question his place in the Justice League. He might think that he's a liability to the team and decide to leave to protect them.
They might be confused at first. Since the Avengers are from a different universe in terms of the storylines. They could be intrigued by the different power combinations and relationships presented 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.
The Justice League's response to the Batman Who Laughs fanfic origin could be all over the place. Some might love the creativity, while others might be critical. It really depends on their individual takes on the story and the character.
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.
They could be shocked. The Avengers characters are so iconic, and their actors brought them to life. Some of the fanfiction might be really far - fetched, like making Hulk into a ballet dancer or something. The actors would be like 'Whoa, that's not what I expected!'
Maybe to save the Justice League from a powerful enemy. He might sacrifice himself if it means the others can escape or defeat the foe.
One possible storyline could be that Batman discovers a conspiracy within the Justice League that endangers Gotham. So he and Robin decide to take on the League to stop it. Another could be that the Justice League is being mind - controlled, and Batman and Robin are the only ones who realize it and try to free their friends by fighting them. Or perhaps there's a misunderstanding where the Justice League believes Batman and Robin have turned bad, and they have to fight to prove their innocence.
One possible storyline could be that Batman sacrifices himself to save the world from a powerful alien threat. He uses his intellect to devise a plan that requires him to give his life. Another could be that he is poisoned by a new villain in the Justice League Unlimited universe, and despite the efforts of his friends, he succumbs to the poison.
Batman's sacrifice would likely have a huge impact on the Justice League. It could make them more determined to uphold justice in his memory. They might also become more cautious as they've lost a great strategist.