It's not "new", there have been references to the Underworld, hell, a spiritual world, an afterlife, magic etc since like east blue and paradise. Oda may or may not have the rules to these things worked out ahead of time, but they've been around in the lore for a long time.
One thing One Piece pretty much never does is explain how a power system works in detail. Where Togashi will make a whole arc based solely on explaining how Nen works, Oda will wait until 500+ chapters in to give the most basic and incomplete possible summary of haki and then leave it open. Call it a flaw if you want.
I think Haki, DFs, Soul, Magic, whatever, probably all have comprehensive details on how they work, they're just not in the manga.