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The blizzard splash screen for all the information about the classes being released until the launch day of diablo 3, each class fills a bubble, there are five classes, but 7 bubbles, one bubble is filled with the diablo 3 release date, that leaves a mysterious bubble unfilled.
Possible that it may be a 6th class, the devs said Diablo 3 would recieve some extremely shocking news before its released, two top devs have been saying there is not 6th class out of no where, almost sarcastically on the offical forums,
A 6th class can have internal testing to rid of bugs, just because players are not playing it does not mean it can not be tested and bug free by the devs/blizzard employees.
If it is a new class, its either a shield user or a Shape Shifter.