Rather than considering the smaller number patterns, which gets awkward due to the way some of the categories tend to blur together, I'm sticking with the "Book #1-10, plus 10" pattern. It doesn't work with Storm Front or Fool Moon, but it seems to work fairly well for the three books after that.
Book #3: All about ghosts, and one ghost in particular, the Nightmare. Harry generates a ghost himself.
Book #13: All about ghosts, and Harry himself is one for all but the final chapter.
Book #4: All about the fae, even called Summer Knight.
Book #14: Apparently all about the fae, and was almost called Winter Knight.
Book #5: We get introduced to the Denarians, and they figure most prominently in the story.
Book #15: We've been told that this will be the next time we see the Denarians.
Following this pattern, we get:
Book #6: Mostly about the White Court, especially the inner politics of said Court.
Book #16: Will the White Court figure prominently in the story? Maybe things with Lara come to a head?