One way to admittedly continue the series; without inventing some massive badass threat out of absolutely nothing, or recycling old ones, would be to set a sequel into the future.
All this is ancient history! That way any new threats would make sense and not be obviously pulled straight out of one's anal cavity. Though they'd really have to do a good job of continuity to keep it in the Warcraft spirit. Which might be asking for too much.
Or a sequel could be less epic and simply focus on one of the unsolved problems we have been introduced to thus far.

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