Do you think it's possible for a mortal to be reincarnated as a Shinjin? If not, then there's a sort of hierarchy of souls. That would mean that Shinjin simply have their own pool of souls that reincarnate into Shinjin bodies, while the pool of souls that can only be reincarnated as mortals is entirely separate.
Do you think that other beings from the afterlife like Enma, the oni, and such are on the same level as Shinjin, the same as mortals, or somewhere in-between?
Could being higher up on this "soul hierarchy" help to explain what was so far up Zamasu's ass?