After all, Game of Thrones eventually shook things up, flipping plots from a couple of different books around to streamline the story.
It’s equally possible that Outlander will eventually be less stringent than it currently has been with the one book plot per season rule.
While changes have been made between Gabaldon’s books and the TV shows, so far the overarching plots in the books have not strayed the course at all.
Of course, there’s always the chance the show pulls a Twilight or Hunger Games and stretches a book or two to make a few additional seasons of the series down the line.