Reverse phantom au: Danny is a ghost that suddenly gains the ability to take on a living form. Instead of starting as a human and gaining a ghost half, he’s a ghost that gains a human half.
Maybe he was native to the ghost zone and had never been human, like how canon Danny had always been alive. Or maybe he was once human and he died at age 14 a long long long loooong time ago. In that case instead of half-dying like in canon, he’s halfway resurrected (main difference between him and a zombie being that he switches back and forth).
At first it may seem underwhelming because to us humans, the dp ghosts are so op and fantastical. But now Danny’s growing/aging (again?), he has the full range and fluidity of human emotions, he can blend in with society in this whole new (or newly returned-to) works.
Anyway I think the reverse au accident would still involve the creation of the portal. When the creators (could still be Maddie and Jack, I haven’t thought it through) first try to turn it on it doesn’t completely fail. There’s the beginnings of a rift, but it doesn’t have the juice to completely breach the veil. Danny notices the disturbance in the atmosphere and floats on up to it. He doesn’t mean to make contact but when he inevitably does, the proto-rift reacts with his energy and opens the portal the rest of the way.
Danny passes out and when he first wakes up he realizes he has jet black hair and dull blue eyes.