If Danny Phantom haunts the Justice League, it could be chaos. Danny's ghostly powers would likely surprise the heroes. He might play pranks on them at first, like making Batman's gadgets malfunction or making Superman's cape flap uncontrollably. But it could also turn serious as they might mistake him for a villainous spirit and try to stop him.
I think it could be Superman. Superman is all about truth and justice and he would likely see the good in Danny. Also, they both have super strength in a way. Danny has his ghostly strength and Superman has his Kryptonian strength. They could bond over protecting the innocent.
Danny could use his ghostly powers to disrupt the technology that some Justice League members rely on. For example, he could mess with Cyborg's systems. His ability to overshadow people might also be a key factor, allowing him to control some of the less - powerful members.
I'm not entirely sure as I haven't read this specific fanfiction. It could be about a disrupted or alternative version of a story where Danny Phantom interacts with the Justice League in some unexpected or 'broken' way.
One common theme is the clash of two different worlds. Danny comes from a world where ghosts are a big part of his life, and the Justice League is from a more traditional superhero world. So there's often a theme of how they deal with each other's differences.
Danny Phantom himself is obviously very popular as he is the main character from his own series. His ghost - fighting abilities and his dual identity make him an interesting addition to the Justice League universe.
Another storyline might involve Danny being wrongly accused by the Justice League of a crime he didn't commit. As he tries to prove his innocence, he uncovers a deeper conspiracy that threatens both the human world and the Ghost Zone. He has to use his wits and powers to clear his name and save the day.
In this fanfiction, perhaps Danny Phantom discovers a plot against the Justice League early on. He then uses his invisibility and intangibility to gather information. His ability to fly and super strength (in his ghost form) would be crucial in battles against the villains targeting the League. For example, if the villains had some sort of kryptonite - like weapon against Superman, Danny could use his powers to either disable the weapon or protect Superman until they find a way to neutralize it.
One possible plot could be that Danny Phantom is facing a threat in his own dimension that is beyond his control. He decides to call the Justice League for help. Maybe he first has to figure out a way to contact them across dimensions. Once he does, the Justice League initially doubts his claims but as they start to investigate, they realize the severity of the situation and they all work together to fight the new and strange enemies from Danny's world.
In such a crossover, Danny could team up with Superman. Danny's ghost powers would be really interesting combined with Superman's super strength. Maybe they'd fight a common enemy that has powers related to both the supernatural and advanced technology.