Diablo 3 Expansion
From Diablo Wiki
Diablo 3 does not have an officially-announced Expansion pack yet, but it's been an open secret since pre-release that Blizzard was planning at least one and probably two or more expansions to the title. Many fans refer to the first expansion as D3X and the hypothetical second as D3Y. The basic game is referred to as "Diablo 3 Classic" D3C or "Diablo 3 Vanilla" D3V, emulating terminology often used for WoW and other RPGs that receive numerous expansion packs that greatly change the function of the original game over the years.
As of mid-2013 it's widely-expected that the Diablo 3 Expansion will be announced in November 2013 at Blizzcon, and the Diablo 3 devs have made several comments strongly hinting at this.
Josh Mosqueira: Blizzcon is going to be awesome this year!
There's much speculation about when D3X might be released, but clearly the pre-release target of 18 months after D3V is out of the question. In recent years Blizzard has required 20-28 months between expansions for WoW and Starcraft 2, and has generally revealed these games 12+ months before they were released. Blizzcon 2013 comes 18 months after Diablo 3's release, which makes summer 2014 seem the earliest feasible target date, with many fans expecting D3X in late 2014 at the soonest.
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: 
- Leaked Release Schedule - MMOGamesite.Com, 2/12/10