The Riftborns. Of course came from a universe , which is completely different from the ES universe.
So the ES universe , may be a multiverse.

Though the multiverse theroy we have now , string theory and M theory , extra dimensions curl and hide in extreme tiny space , and the shape of extra dimensions decided the form of a universe.
Consider those theories , the Riftborn universe may just have different shape of the extra dimensions.

According to the turth that the Endless is a creature kind like they are made by virtual reality.
So ,the Endless may be just a kind of a projection of a higher dimension , from a four-dimensional creature.
Or it is a holography of a lower dimension , from a two-dimensional creature ?
So that , Endless may be a two-dimensional or four-dimensional creature ?

Also konwn that the Endless defeated the Losts ,then the Losts just gone disappear.
And then ,the Endless got the ability of virtual reality , which were once the ability of the Losts.
So , the ability of virtual reality , if that is just a kind of a holograpic or projection of a lower-dimensional or a higher-dimensional creature...?

How did a three-dimensional creature defeated a lower or higher dimension creature ?
The Endless may once three-dimension , so were they found a way to reduce or increase their dimension ?

emmmmmm...

A cosmos which is full of energy , in the endless inflation , when the engery bang , create multiple universes. 

Such is a little like the image of the morden physics tend to draw in future. 

Just have thought about somethings deeper in the game designs combine with some morden physics theories.

This may be the Sci-fi and its fascination. emmm

They may truely exist , somewhere multiverse.