Pat here: As readers, we probably started talking of world building in fantasy, where the monsters had to have their own society, their own faults and strengths, and the setting had to make them real.
But now, as series become more popular in other genres, we’re all engaging in some lengthy world building. When I first started writing about the Magical Malcolms about ten years ago, I was simply creating a world of characters with special psychic abilities. Yes, the era was Georgian, but the setting wasn’t as important as how the characters felt about being weird in a normal world.
About the same time, Jayne Ann Krentz started mixing her Arcane Society paranormal romances into her historical world. JR Ward turned vampires into contemporary romance, and suddenly, any kind of series was hot and could branch off in any direction as long as the reader could buy into the richly-created worlds they built.
I’ll admit, I love the world building, but it’s not easy when limited by history—or a related series.