We still have another year or seasonal content. Assuming they can’t multitask (which is often the case) we won’t get D3 until at least 2022. Maybe even later if Bungie thinks these D2 expansions will keep making money.
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D3 May never come out if they can figure out a formula that works for D2. They said in an interview that the main issue is the size of the game and if they can effectively remove old content as the story progresses.