I'm not sure if this has been discussed on here before, but the end of the novel "Lord of Souls" has some new info on the Hist.

************************************************Spoilers Obviously**************************************

So there are these trees on Umbriel that are sentient and able to communicate with Argonians. They are also capable of communicating with the Hist in Blackmarsh and can create and shape life the same way the Hist do. The Argonian Mere-Glim speculates that they may be cousins of the Hist. Later they turn out to be even more similar to the Hist than he originally thought. The author stops short of telling us that they ARE Hist, but we can at least surmise that they are very very similar. They also have some pretty impressive powers. They control and manage Umbriel and seem to be the force that controls its movements. They are capable of recycling souls into new bodies (using the Ingenium of course) and even have the power to move Umbriel into different realms.

The really interesting part comes at the very end of the novel. After a barrier created by Clavicus Vile is finally destroyed, the Hist-like-trees are finally able to take themselves and their creations home. They travel to some realm that is not Mundus but also is not the realm of any Daedric prince. It is vast forest full of sentient trees and Mere-Glim sees a tall spire on the horizon (maybe a Tower?).

Maybe this is the home of the Hist as well?

What and where is this realm? The only thing I can think of is that it might be a realm of Oblivion that does not belong to any Prince, some what like the Soul Cairn.

But with the addition of these new realms (Soul Cairn and Talking-Tree-Land) doesn't the conception of the Wheel kinda fall apart? If we have the 8 spokes and 16 realms in between, where do these additional realms fit in?