I am now in the process of writing the first synopsis for the third TERRA HEXA book.
This got going only once I had figured out a suitable "theme" for the story, something that can hold the plot and its threads together. And the theme is... but that would be telling. ;-)
I like to think of a novel in terms of levels:
-Theme (i.e. "idea")
... in that order. Theme and plot are more important than character arc. While characters matter, they alone do not carry a story. But that's just my opinion...