A Blizzard Representative confirmed unofficially plans for their second World of Warcraft expansion, and that they will attempt to carry on the goal of an expansion per year.  No promises or guarantees that it will be so, but that’s the goal.

Nela: Do you guys (Blizzard Ent.) have any preliminary plans for the expansion after The Burning Crusade?

Nethaera (Blizzard): Yes, we are working on plans. No, I can’t tell you what they are, how far they are or what timeline we are looking at. Sorry. 🙁

Nela (Fan): In the some of the developer interviews it was mentioned that Blizzard was thinking of releasing expansion quality content on an annual basis after TBC.

Nethaera (Blizzard): As far as I’m aware that is still a goal. Keep in mind that there is a level of quality we want to maintain with each release as well so timelines are subject to a bit of flex.