Danny being immortal didn't allow much leeway in life. Turned half ghost while in puberty did allow him to keep growing until it ended, in both powers and body. In fact, he grew exponentially, taller then his dad, as powerful as clockwork, if not more. For a bit he didn't know if he would stop. But he did, he knew he did. His friends and family kept growing and changing while he seemed to be stuck in time. The world had kept spinning without him and he was stuck looking 20 for eternity.
He remembers holding Sams hand when she fell I'll, surrounded by wires and a beeping monitor. Wheeling Tucker around when he broke his hip and his children were to busy to visit him. Planting Jazz's favorite flowers along her grave, knowing she left the world too soon.
Ghost took years to form, if they even formed into a ghost in the first place. So he waits, he's not exactly dead when the dead exists, but it's always lonely when your closest lover on dies.
Instead of focusing on that, he travels instead. Danielle had got bored of exploring the cosmos so he joined he in expiring the infinite realms. There's portals that leak into different dimensions. That he finds himself hopping through. Creating new bonds along the way and having as fun as possible.
He changes things around to make the ghost zone as comfortable as possible. Being crowned king has its perks and he waits patiently until his friends arrive
