Unfortunately for World of Warcraft fans, that day probably won't come for a while. While the original World of Warcraft is over 6 years old, the game continues to update frequently and Blizzard is focused on releasing expansions for the game. The expansions are The Burning Crusade (2007), Wrath of the Lich King (2008), and Cataclysm (2010). Each expansion updates the entire game's graphical capabilities and adds significant add-ons for players. The Burning Crusade and Wrath of the Lich King both add 10 levels to the original level cap while Cataclysm raises it by 5. The Burning Crusade and Wrath of the Lich King added new worlds/continents for players to explore while Cataclysm added new territories on the original continent. Aside from levels and areas the games add new races or classes to go along with hundreds of new quests and dungeons/raids. While these are still termed as expansions they definitely add enough to the original to be considered new, in a sense. However, in terms of a whole new game Blizzard is defiant and continues to state that they have no plans at the present of creating an entirely new game, as evidenced by their recently released expansion Cataclysm.

This doesn't seem to stop the perpetual rumour mill. Whenever Blizzard says they're working on anything fans immediately believe it to be a WoW sequel. An example can be found here:
http://www.bit-tech.net/news/gaming/2011/02/14/blizzard-titan-mmo-is-not-world-of-warcraft/1