One thing I think writers sometimes don’t think about with their fantasy worlds (and perhaps worlds in general to a lesser extent) is what that world is about. About? How can a world be about something? It seems contrary to all common sense. After all, a world is just a place for your story to happen, right?
Right, but because it is the place where your story happens, it should reflect the themes of your story. It should reflect the tone of your story. It should add extra dimensions to it and provide other points of view on it. It should be the space where things happen that affect what your book is trying to say, and will always be the circumstances in which your character acts. So yes, it can be about something too. Every aspect of your world will grow up from the main ideas on which it is based, whether you have decided on a tough world where nothing is easy, or a world where everything is shrouded in intrigue, or one that is about the classic conflict between good and evil.
So what is your world about? Well, what is your novel about? What themes does it explore? What kind of elements interest you? Are you interested in the complex internal politics of great houses? Ideas of true love? Big, sweeping battles? Do you want to say something about nations and the way they work? Families? Do you want to explore the darker realities of ‘heroism’ in the fantasy sense?
The point here is that if you understand the kind of ‘big idea’ behind what you’re writing, you’re more likely to put together a world that allows you layer after layer of commentary and exploration on that theme, rather than running off in directions you don’t have much interest in. You can create it, secure in the knowledge that everything you put in has meaning, and isn’t just there as filler. Better yet, you can be sure that the decisions you make about your fantasy world contribute to the overall story, so that they aren’t just background anymore.