In my quest to blast out a full novel in 7 months, I do have one advantage: The framework of a world pre-built with some level of mythos, origin and society. A couple of years ago when my daughter started to really get into The Walking Dead, I wondered what kind of havoc such a world would play with the natural order. The zombies in TWD are slow and mindless.
What does that do for existing predators like wolves or big cats? Wouldn't these beasts suddenly find plentiful food in the form of slow creatures? And, so long as they didn't like something horrible - such as liver - wouldn't the bountiful food supply mean an explosion of growth from the beasts?
It seemed logical and formed the basis of a novel concept. The tagline also arose from this: "It turned out the zombie apocalypse wasn't the end after all. Mother Nature had other ideas." So I whipped out a short story about a future where natural scales are balanced against the rise a new dominant species by introducing an old one. It was a nod to my daughter's interest and a good discussion piece for us. I wrote it, she read it, we talked and I left it be. Never did I think to revisit it. Thankfully I have a mind like a steel trap: Rusted shut. Part of my decision to enter into this mad march was propelled by the fact that I had something ready to go, indeed something I'd already written for her. If I'd had to create a concept and world from scratch, it would have burned weeks, even months, off my timeline. Thus, I only needed expand it by about 20x. Simple, really.