Although I'm not too sure with what's going on with fanon/canon, here's my ideas about Fae King DP AU.
*While I'm not sure HOW Danny gets into the Fae world, it's one; by accident, and two; can be replicated on the modern side.
*The Ghost Zone is connected to the Fae world, but most of the younger ghosts don't know about it and the older ghosts don't really care about it.
*In addition to this, the Ghost Zone has a BUNCH of different types of ghosts; classification, then subtype, and maybe species. I'm working on it.
*The Fae world has a moderate population of humans, a large population of elves, and most species that are classified as magic; dwarves, nymphs, ect ect. Currently, though, the elves are split in two factions; the magic boys and no magic boys.
*Magic is REALLY finniky. Think of magic users similar to force wielders, but more karmic and if you mess up REALLY badly you lose limbs/family members/memories/important stuff.
*Humans try VERY hard to be neutral/not immediately murdered. It's... a thing. History made them the underdogs, and the underdogs have yet to win.
*Danny, when he's there, ends up getting kinda stuck in this weird half-ghost half-human kinda thing, unable to stick to one full morph. It's the magic messing with his ghost core. Sometimes he's a bit more ghostish: ie, floating for small periods of times, accidently walking through someone, appearing transluscent, attracting small left-over ectoblobs or accidently freezing stuff.
*He does NOT have a good time. He lands in some sort of forest, with no civilization in sight, while trying to deal with the weird magic sickness. He's stuck alone for a while, and his powers are going haywire.
*Eventually he stumbles into a small mixed village. Danny's first thought is "oh. crap." and his second thought is "toto, I don't think we're in kansas anymore." A bit of clothing goes missing the next day and he's just kinda scouting out the rumor mill and stuff.
I'll add more bits when I'm done worldbuilding.