I thought that people who die in the real world go to hell or soul society (if you're a plus). I think of ryokas as lost souls and/or people in purgatory. As for shinigamas, they are either nobles or pluses (like Byakuya) or ryokas (Renji) both of whom have high spirit power. Those ryoka or pluses with strong/high spirit energy age and get hungry albiet at a slower rate than living people. Where they go when they die...no idea.

my 2 cents