Concerning the Po-Raiders being involved, I was thinking of something nearly identical (again) - except that instead of all of them playing a part, it's ol' Ahkmou at it again. Little did anybody know, he was a Dark Hunter the whole time!
Jokes aside, the idea of the Shadowed One dwelling beneath the center of the island would be an insane (and satisfyingly disturbing) twist in a future arc. My only issue with it is that the Dark Hunters feel more suited to be based on another island somewhere else, closer to areas of better opportunity.
Honestly, it wasn't my favorite, either.