I have a confession to make. I was a bit worried about how thin my plot was for Beyond the Black Veil, because I didn't really have a link to my other two books figured out as yet. There was something intrinsically missing. Something that had to hit, not only me on the head, but really throw my readers when I revealed it toward the end, at the climax. I actually have not gotten to page 200--but I'm close, about 2 pages away! And I'm about at the climax point of the book, and yet I needed the crucial moment that brings everything together, that makes the reader go "Oh. My. God!" hopefully.
Figuring these things out is something that merely comes with time. I know, from the years and years--well, okay, decades!--of writing (which includes all my practicing years, the "author in progress years"), to know that the A-Hah! Moment will usually arrive on a silver tray directly into my brain and I'll be so exited I might jump up and down--but can't because I'm either driving the silly bus around, or people are watching me.
What I usually do is find a place to sit down and grab a pen out of my purse and a pad and scribble these ideas down before I loose them.
Many times it comes to me at night. So, a number of years back I got myself one of those mini-recorders. These are especially great for those madness at midnight moments when you wake up with the idea that you simply can't write down (eyes are bleary, and maybe you might wake someone up). Grabbing the mini-recorder I'll rush into the kitchen or bath. Yes, maybe I tinkle while I make the verbal notes, but hey, I like to multi-task!
The important thing is to get those thoughts down quickly. Revise later, because you will revise. Especially after the Midnight Madness moment. That's why I posted the picture above. It reminds me of waking up and having my characters reveal something to me that I had not known up until this point.
That's what I love about writing. You have a lot of fun creating those moments, and honing them down into something people will read and enjoy.
And, nope. Not gonna tell you what came to me. It would really spoil the plot and ending.