So, not so long ago, the Elder Scrolls studio trademarked the name 'Soulburst', you can read more about it here; http://uk.gamespot.com/news/elder-scrolls-studio-trademarks-soulburst-6414197.
Now, I may be wrong (fairly certain I'm not), but isn't the Soulburst what happened in 2E 579?
"An arcane explosion of energy in the Imperial City set off mystical aftershocks that swept across Nirn. Mages died or went mad. Supernatural abominations from the plane of Oblivion, the Daedra, appeared in greater numbers than ever before. The constellation of the Serpent grew so large that it dominated the night sky." ~ http://elderscrolls.wikia.com/wiki/Timeline.
So, I'm guessing this is going to be an expansion to the Elder Scrolls Online, it seems very likely that it would be, considering the fact that there's a huge focus on the Imperial City (and the fact we can't visit the Imperial City at launch), so it could be a very good excuse to open it up and let all 3 factions go nuts for it, it'll be flooded with Daedra and whatnot, too.
But, reading from here; https://www.facebook.com/notes/the-elder-scrolls-online/introduction-to-the-lore-of-the-elder-scrolls-online-en/486381251371913 it says that the Soulburst happened 4 years before the events of the ESO. So, how would that work?
Anyway, I just wanted to raise attention to it, I haven't seen any threads about this as of yet, so I thought I'd open it up and create some possible speculation about what it is.
Thanks for reading!