Probably won't happen, as it's already easily doable by datamining a complete list of quest IDs available for your class/race (eg. from wowhead) and comparing it against the list returned by GetCompletedQuests() to determine which quests you haven't done yet. The reason GetCompletedQuests is so useful is that it provides information that can't be obtained through the API or through any kind of datamining.
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