SPECULATION The Netherworld of the Force.

I don't think we will see it, but it would definitely be interesting. What role would this Netherworld of the Force play? Would Rey use the energy of this netherworld to defeat Kylo Ren?

 

We get a glimpse of this, perhaps, in Claudia's Gray's story in From A Certain Point Of View. Potential spoilers for that book. It seems that, for Qui-Gon, the "netherworld of the Force" is oneness with the Force. It means becoming indistinguishable from the life force around you, actually losing your identity and becoing part of that energy, so that to manifest as a Force ghost you need to fracture yourself from it momentarily.

So in canon, the "netherworld" here is perhaps less of a place than a state of being.

The best visual glimpse of this might actually be that sequence in TLJ where Rey sees life, death, peace, violence, and the balance between them. A poetic counterpoint of images from which we infer a life-force, rather than a contrived place.