https://twitter.com/TESOnline/status/624693648333099008
Assuming they stay consistent, assume we'll see the second major DLC pack sometime in very early 2016.
Updates:
Orsinium, our next #ESOTU DLC game pack, will be available in Fall 2015 with around 25 hours of new content. #QuakeCon
https://twitter.com/TESOnline/status/624705295768489984
The Thieves Guild & Dark Brotherhood will be two separate #ESOTU DLC game packs, scheduled for release in the first half of 2016! #QuakeCon
https://twitter.com/TESOnline/status/624705490979815424