User blog comment:Raylan13/Elder Scrolls Online Community Q&A/@comment-74.113.2.90-20140430174244/@comment-3124449-20140501045850

Not announced yet. Given their recent release pattern it would most likely be released in 2016 or 2017, but Bethesa could potentially be forced to postpone it by ZeniMax to maximize profits from TES fans playing TESO. My guess is somewhere between 2017 and 2020, depending on the popularity and longevity of TESO.