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.
There's also a story titled 'When Danny Saved Sam'. In this fanfiction, Sam gets trapped in an other - dimensional realm, and Danny embarks on a thrilling adventure to bring her back. It's full of action and some really sweet Danny - Sam moments.
His parents might be really confused at first. They've been hunting ghosts, and suddenly their son is a baby again. They'd probably be worried about his well - being and try to figure out what happened using their scientific knowledge. Jazz, his sister, would likely be very protective. She'd take on a more motherly role, making sure Danny is fed, clean, and safe.
Danny Phantom is for sure the main character on the saving side. On the Justice League side, all the classic members would probably be there. Superman, being the symbol of hope and having super strength, heat vision etc. Batman, the Dark Knight with his gadgets and detective skills. Wonder Woman with her Lasso of Truth and superhuman strength. The Flash, who can move at incredible speeds. Green Lantern, who can create constructs with his power ring. And Martian Manhunter, with his shape - shifting and telepathic abilities. They all could be in danger and Danny would have to figure out how to save each of them based on their unique powers and vulnerabilities.
There is definitely some out there. One type of fanfiction could be about a member of the Bat Family encountering Danny Phantom and being both intrigued and cautious. For example, Batman might initially be suspicious of Danny's powers but then come to realize they could be used for good in his fight against crime in Gotham. Another aspect could be how Danny's friends from his own series interact with the Bat Family members.
Danny Phantom might adjust by using his powers in new ways. For example, he could use his invisibility to help Catwoman during her heists or surveillance. His ghostly powers could give them an edge in Gotham's dangerous underworld. He may also learn new skills from Catwoman, like combat techniques and stealth. Over time, he'd start to see Catwoman as a new family member and adapt to the different rules and lifestyle of living with her.
Danny might react with panic at first. After all, he has been hiding his identity from his parents for so long. He could try to escape from them, using his ghost powers in a desperate attempt to get away before they can do anything drastic.
One common theme is family acceptance. Since Danny is a half - ghost, his family has to come to terms with it. For example, in many fics, his parents go from being ghost - hunters to understanding and protecting their half - ghost son.