There's Heaven, Soul Society/Hueco Mundo, the mortal realm, and Hell.
It might borrow from the Buddhist philosophy of reincarnation, your presence in each realm determined by the lesson you need to learn before reaching Heaven--the equivalent of Nirvana. Sentencing to Hell may or may not be permanent, maybe it just takes an extremely long time for evil to be burned away from a soul (but then, maybe not).
Think of Hueco Mundo and Soul Society as two sides of the same coin, or Yin and Yang. Where Soul Society exists similar to the mortal world, Hueco Mundo is the dark reflection in the mirror, a feral opposite where tormented souls (since we've seen spiritual distress result in Hollows), remain in a bestial predator/prey state.
Shinigami perhaps exist as the answer to Hueco Mundo, since Shinigami possess the ability to purify Hollows into Pluses (that name for normal souls that Kubo immediately discarded).
But anyways, Hueco Mundo might have it's place as Yin to Soul Society's Yang, the mortal realm as a proverbial fulcrum between the two, with Heaven and Hell on the extreme ends of the spectrum.