And even before we mention the question, we'll answer it for you: yes, Blizzard plans to release an expansion for every year that passes. But as long as it remains in the planning stages, such an answer to the question "is Blizzard going to release another expansion soon?" is moot.
While Blizzard is not keen to tell what exactly they do have in their plans, there are plenty of other content available right with your current World of Warcraft build that isn't finished. One such content is the Emerald Dream, which according to some players, its content has changed inside one of the MPQ files in World of Warcraft.