Well, Danny could use his ghostly powers to play all sorts of tricks on the Justice League members. Maybe he turns Superman invisible for a while just to see his confusion. Or he makes Batman's utility belt float away when he's in the middle of a fight.
In the 'Danny Phantom Pranks the Justice League' fanfiction, Danny might use his intangibility to sneak up on Flash and steal his speed for a moment, causing Flash to trip over himself. He could also mess with Aquaman by making the water around him act strangely, like forming into odd shapes or moving against the current.
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.
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.
Well, he could bring his unique ghost powers to the team. His intangibility and invisibility would be great for stealth missions. And his ghost ray could be a powerful offensive weapon. Maybe he'd also have some funny interactions with the more serious Justice League members like Batman.
In a Danny Phantom and Justice League crossover fanfic, Danny could team up with Superman. Maybe Danny uses his ghost powers to help Superman when he's facing a powerful ghost - like enemy that the Justice League can't handle with their normal tactics. It would be a really cool combination of the two universes.
Obviously, Danny Phantom is one of the main characters as the title implies he's the one doing the pranking. And then, members of the Justice League like Superman, Batman, Wonder Woman, Flash, etc. would be the ones being pranked.
In a 'danny phantom vs justice league' fanfiction, Danny could use his ghost powers to surprise the Justice League members. His intangibility and invisibility might give him an edge at first. Maybe he'd startle Superman by suddenly phasing through a wall. However, the Justice League has a vast array of powers and skills too. Batman could potentially figure out Danny's weaknesses with his detective skills. And Wonder Woman with her combat experience could pose a real threat to Danny in a face - to - face battle.
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.
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.
One possible plot could be that Danny Phantom's ghostly abilities are detected by the Justice League. They initially view him as a threat but then realize he can be an asset. Danny could help them with foes that have intangible or otherworldly powers. For example, against an enemy that can phase through walls or possess people, Danny's experience dealing with ghosts would come in handy.
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.