From what I recall, Aetherius is the source of all magicka. I don't know the details, but everyone else seems to be filling them in, albeit a different story from each poster as per usual.
I do think it would be pretty cool to actually explore Aetherius in a future Elder Scrolls title, even if it's not part of the game's main plot line. Although considering all the custom scenery, textures, props and creatures that would go into building something as important to the series as Aetherius, if we ever do get to visit, it probably will end up being part of the main plot line.
We've seen Hellblivion, now it's time for Heavetherius!