What if the Soul Cairn is simply that: a Cairn?
This is more a hypothesis than a proper theory, but when I thought of it, I couldn't not post about it.
For those unaware, a Cairn is essentially a pile of stones, often used to mark trails or burial sites. They appear frequently in Skyrim itself, often with a long cloth or banner serving as a weathervane. This hypothesis is that the Soul Cairn is very much like one of those, a plane(t?) of Oblivion formed by trapped souls, stacked higher and higher as time goes on. This, theoretically, could ultimately end up forming a god in and of itself, functioning as a Crux of Transcendence.
The real question, though, would be that if this is the case, what does that mean about the Ideal Masters? Are they merely some of the lowest, most foundational souls in the stack, or are they themselves the ones doing the stacking? Or are they both? Did they intentionally start the Cairn, or did it form naturally?