Either way, at some point, writers must commit to their new idea. Here are some of the steps I go through when deciding to spend the next months (or year? Years, even?) with the next novel.
What else is already out there?
What is my main character's journey going to be like?
So. The journey. There are lots of classic journeys that main characters go through. Will it be a classic hero's journey? How will the MC change and grow? What are the stakes? Are they important enough? When I ask these questions, I try to feel if this is a story I actually care to create, watch unfold, and be involved in.
I am a genre-hopping writer. CONTROL and CATALYST are firmly in the realm of science fiction, defined by the genetic manipulation and near future realities within it, but I've also written historical, urban fantasy, and have high fantasy and even more historical on my list of things to write. I've lived in YA but also dabble in MG. I always try to read lots of books when I'm entering into new territory, so I can get a feel for how other authors tackle them (also known as the most fun homework, EVER.) I know we hear the common phrase "Write what you know," but part of writing is about using our imaginations to expand into what we're unfamiliar with. However--if it's so unfamiliar that you're deeply uncomfortable in that territory, well. That's telling.
This is the biggest question of all. After all this work, am I still in love with it? Because to tell you the truth, I've never written a novel where my heart didn't flutter at the very concept of it. I can't explain why, but the spark sometimes disappears after fleshing it all out. Those books have never been written. However, if I'm still swooning over the idea, then Spock isn't allowed to destroy it all. I open up my Scrivener program and start writing page one. :)
How familiar am I with this genre?
Can I speak the language?
This is really a research/brainstorming question. Before I can write a single word, I have to know what my characters are going to sound like, what they wear. I have to know what the buildings look like, if the food is spicy. I have to know about the politics of the time and the historical background--and I'm not just talking about writing historical novels. This is true for epic fantasy, and futuristic thrillers, and contemporary. If you can sit inside your world and really see it--you're ready to live there a while.
Who else lives here?
World building is one thing, but who else lives there? These are the people and animals and creatures that can really bring vibrancy to your world. A main character is nothing without the supporting cast. So I have to meet them and be just as entranced with them as the MC and world.
Am I in love with it?