Diablo 3 Expansion
From Diablo Wiki
Diablo 3's first expansion pack, Reaper of Souls, was officially revealed on August 21, 2013, from the Gamescom show in Cologne, Germany. Key content additions include the Crusader class, a full redesign of the Paragon system, the Loot 2.0 redesigned item system, new types of end game content including Loot Runs, and much more yet to be revealed.
Prior to the full reveal, Blizzard offered numerous hints and teases that something big was coming up at Gamescom and then much more would be revealed two months later at Blizzcon 2013[].
The existence of a Diablo 3 Expansion pack was not a surprise, since the developers have never made a secret of their desire to create at least one. And fans were calling the D3V (vanilla) or D3C (classic), with the expansions designated as D3X and D3Y, long before Reaper of Souls was revealed.
Reaper of Souls will be released on March 25th, 2014. The closed beta test began on or around December 14th, 2013.
Diablo 3 Expansion Rumors
Most fans simply assumed Diablo 3 would have an expansion, as virtually ever successful video game does, especially in the RPG genre. Furthermore, pre-release the Diablo 3 devs answered questions on a possible expansion by pointing out that Diablo 2 only had one expansion, and that this was something of a lost opportunity as long as the game remained popular, thus fueling fan speculation that Diablo 3 would see two (or more) expansions.
This suspicion was all but confirmed by a Blizzard Product Schedule leaked in early 2010, which you see in the image to the right. It listed Diablo 3 as targeted for the second half of 2012, with the first expansion 18 months later, and the second 18 months after that.
Jay Wilson: Right now we’re just focused on getting the game done. We have ideas, but we don’t know. This one’s a handful. All I’ll say is that the previous Blizzard games just did one expansion, like Starcraft and Diablo II, seemed like a missed opportunity. We’ll make more D3 content until we feel like we don’t have anything more to say.
Are there plans to make more regular expansions? Each 18 months?Jay Wilson: WE haven’t really thought about it yet. All of our time/energy to make D3.
Post-Release Content Plans
The unacceptable (to many fans) state of Diablo 3 upon initial release spurred several major content patches in the months after release. This was not what the developers had planned on, and most fans assume that everyone working on patches necessarily delayed the expansions well beyond the pre-release plans of one coming every 18 months. (An announcement at Blizzcon 2013 would come 18 months after D3's release.)
Jay Wilson spoke on their post-release patching plans in August 2010: 
Announcing Reaper of Souls
In early 2013, Reaper of Souls was copyrighted. In addition, an image was posted on an unlinked page at the blizzard website, saying: "And death shall spread its wings over all." A few weeks after, the expansion blog was up and the expansion itself, confirmed.
- Leaked Release Schedule - MMOGamesite.Com, 2/12/10