I'm a total idea junkie. For me, the initial spark of inspiration is the absolute most exciting part of the process--followed closely by drafting those opening pages.
When I first became a full-time writer, I was always getting distracted by shiny new ideas. I even tried to fool myself that I could actually write multiple projects at once--the project that had hit the sluggish middle AND that brand-new idea.
Yeah. Maybe that works for some writers, but it really doesn't for me. All that happens is that I start a hundred new projects and nothing gets done. So I started keeping notebooks. Big notebooks that pile up next to my computer at home, small notebooks I can stick in a purse and carry with me. I write down every single possible new book idea that goes through my head, then get back to my current WIP.
The cool thing is that when it's time to start a new book, what you've basically got is an entire notebook (or two or three) of writing prompts. Do some freewriting using your notebook(s) to help guide you. Keep flipping through your pages; figure out ways to connect seemingly unconnected ideas. It'll take your initial-stage brainstorming to an entirely new level!