I love all of your ideas! It would be so cool to have an "afterlife" world. I imagine it having a colorful Dia de los muertos theme. There is an animated movie called The Book of Life that can be a good reference.
Anyway, at this point, to be honest, I´d already be happy if they fixed some of the issues with ghosts. They´re basically normal sims with absolutely no difference in the gameplay. But what gets to me the most is how they keep calling my sims and keep showing up in the relationships menu. Also my sim had a baby at the hospital the other day and the doctor was a ghost. Meh. In the end there is no point in trying to bring them back to life because they're still so present after they die.