Focus or the lack there of can kill your book before you’re even finished. The lure of other ideas can be tempting but if you let yourself be distracted you may never finish what you are currently working on. That amazing brain child will end up still born, never realizing its promise.
As a writer you will have other ideas. It’s a given. That is why a good portion of us write; to rid our brains of the clutter that accumulates. Write down your ideas. Keep a notebook. Just don’t let the idea of something new and shiny lead you away from your current project.