For those authors more in the beginning stages of creating their books, I understand what you're going through. I've been a hybrid planner for many years, part seat-of-my-pants and part plotter (knowing two things: as many as seven main plot points and up to sixty possible scenes). That's worked for me the three times I completed NaNoWriMo. But that resulted in three stand-alone novels in draft form, not Books 1 through 3 of the same series.
A series is a different animal. It takes forethought, much more so than for a single novel. To write a series, I needed a setup and a cast that I would enjoy working within and with for years. Also the wherewithal to add new characters. I needed an overarcing storyline that ran through each of the books (like murder mysteries solve the murder in each book, except I'm not writing murder mysteries), yet each book within the series was a self-contained unit. It's a major undertaking. You don't want to end up in the middle of Book 2 with no way out for your characters and no plausible plotline while staying true to the setup given in Book 1. Contemplating a viable series was daunting.
So I've written books to help me in plotting a single book, which I would apply to preparing my series, projecting even three books into the future. My latest publication was the BUILD-A-BOOK Storytelling Checklist which contains 110 items to make note of when plotting a book. I took gems amassed from myriad books on plotting, then took the best of the best and arranged them chronologically. My other releases that help me in plotting are my four volumes of quotations. To me, they represent writing prompts. When I'm stuck in the plotting process, one of the things that can help me the fastest is to read a few pages of quotes. Invariably I'll hit upon a theme, about which I can talk/write forever (spanning three or more books even, I hope), or I'll find a few quotes that evoke a rush of emotions. Then I play around with those themes and emotions, brainstorming and/or mindmapping to see where it leads me.
Invariably I have to let it simmer on the back burner of my mind. Then, in the shower, unloading the dishwasher, mowing the lawn or whatever mundane chore I'm doing, I'll have a moment of inspiration where it all falls together. It's wonderful when that happens.
At this magical moment, I have to immediately write everything down. Usually as a letter to myself, stream of consciousness fashion, just getting it on paper or into my laptop before I forget a thing. Afterward, I work through the Snowflake software, from a logline to a fully fleshed-out plot and scenes list.
Anyway, that's my process. And I've started Book One of my proposed series in my daily hour or two of downtime, with future books in this series becoming more and more apparent. I'm proof positive that the muse visits those while writing (not just waiting).
Hope this helps someone else get over the plotting hump. Take whatever resonates and toss the rest. Happy plotting!
Author, Blogger, Freelance Copy Editor