AU where Phantom has just been given his first assignment by the Observants to check out a possible GZ breech and he’s so excited because he’s going to protect the ghosts from the humans and even the humans from the ghosts and even though the Observants say that it probably won’t actually work he just has this feeling and even Clockwork was--
and then the Portal rips into the GZ, right where Phantom is floating, and he feels the searing heat of electricity, and for the first time in his afterlife he is made aware of the actual cold of the ectoplasm around him and then he stumbles out of the portal, not quite himself. He knows things that he shouldn’t, about space, and a thing called high school, and humans, but these things aren't new but they are, and his light weight seems so weird to him now and there’s a voice there that isn’t quite his but it is. He looks at his toxic green eyes in a daze. are they his? who is he? He is Danny Fenton, wait but isn’t he Phantom? No, he’s something new. He’s Danny Phantom.
And the Ghost Zone has lost a promising resident. Ghosts go to check on Phantom who they haven’t seen in a while but where he should have been investigating was a portal, forcing itself into existence, staying there. And of course they’re angry, the Phantom they once knew has been almost entirely overtaken by a human teenager and it’s wrong. Walker tries to punish Danny Fenton, the human responsible for the death of Phantom (well, as dead as you can be when you’re already a ghost), and he needs to pay for that. Skulker wants Danny dead and on his wall because maybe if he removes the human pelt then Phantom will be free. Others come to the human realm to see the human who trapped a ghost inside of him to gain his powers.
Danny is different after the accident. He’s more protective, slightly more naïve, and a bit more passionate. He had to relearn some everyday human things (”geez Danny, when was the last time you brushed your teeth?” “brush my teeth?”) Sometimes he gets snippets of memories filled with faces he doesn’t know. When he went to the GZ, there were parts that seemed oddly familiar. The studies of human math and science don’t seem quite right to him. He wrote it off as a side effect of the accident with the portal, but there were parts of himself that he lost, and although he didn’t quite know it, he gained more than just Phantom’s powers.