One idea could be that Danny Phantom gets transported to Poison Ivy's world. He has to deal with her plant - based powers while trying to find a way back home. Maybe he teams up with her against a common enemy who is threatening both the Ghost Zone and the natural world.
In this fanfiction, the gods in Percy Jackson's world are intrigued by Poison Ivy's power. They bring her to Olympus to study her. Percy is assigned to be her guide while she's there. Along the way, they discover that there are similarities between the magic in Poison Ivy's world and the divine powers in Percy's world. They start to experiment with combining their powers, like Percy using his water powers to help Poison Ivy's plants grow in unexpected ways, and Poison Ivy using her plants to create shields for Percy during his training sessions.
A fanfiction could also explore a scenario where Peter Parker gets some ghostly powers similar to Danny's due to an experiment gone wrong. He then has to learn how to control these new powers while still being Spiderman. Danny could come to help him understand and master these powers.
One idea could be that the Joker somehow finds his way into the Danny Phantom universe. Maybe he teams up with one of Danny's villains, like Vlad, thinking they can cause more chaos together. They could plan to use Danny's ghost powers for some really wild and dangerous stunts across both the human and ghost worlds.
A more complex fanfiction could be about Starfire getting stuck in Danny's world. She has to adapt to living in a world where ghosts exist and not everyone is as friendly as in her own. Danny helps her understand the rules of his world, and in the process, they develop a strong friendship that could potentially turn into something more romantic as the story progresses.
They could also form an uneasy alliance. Danny's powers can be used to protect Poison Ivy from those who want to harm her for her actions. In return, Poison Ivy could use her plants to create a safe haven for Danny when he's in the human world and needs to rest or hide from his enemies. Their relationship could be complex, full of mistrust at first but gradually evolving into a mutual respect.
Another fanfiction concept is that a character from The Walking Dead dies and becomes a ghost like in Danny Phantom's world. Danny has to help this new ghost come to terms with their situation while also dealing with the zombie apocalypse around them. This could explore themes of death, acceptance, and survival all at once.
One idea could be that Danny Phantom accidentally crosses paths with the Grim Reaper. Maybe the Grim Reaper is after a ghost that Danny has befriended, and Danny has to convince the Grim Reaper that the ghost has a good reason to still be around. For example, the ghost could be helping Danny protect Amity Park from other more malevolent ghosts.
One idea could be that Danny Phantom gets transported to the Spider - Man universe. He could use his ghost powers to help Spider - Man fight villains that are a bit more supernatural in nature, like Mysterio when he's using his illusions. Danny could phase through objects to avoid attacks and then surprise the bad guys.
One idea could be that Danny, using his ghost powers, somehow gets transported to the Dragon Ball Z universe. He might initially be mistaken for an otherworldly being. He could team up with Goku and the gang, and use his intangibility and invisibility to help in battles against foes like Frieza or Cell. For example, he could sneak up on enemies and disrupt their energy blasts from within.
One idea could be a crossover where Danny Fenton, as Danny Phantom, accidentally gets sucked into the Devil May Cry universe. He teams up with Dante to fight a new type of demon that is causing chaos in both their worlds. Maybe this new demon is using some sort of ghost - like powers, which Danny can help understand while Dante provides the combat expertise.