I feel like for me, my ideas are quite deliberate. For my first book, I wanted epic fantasy dialed down to an easier level, so it became contemporary fantasy (less world building, single protagonist). That was mostly due to inexperience – I wanted an ‘easier’ book to practice on.
My next series was commissioned, then came an urban fantasy academy series. It was literally ‘this is selling, create an idea to fit’. My publisher, my series buddy and I brainstormed around that criteria until we found a good fit.
My current project is more a passion project. I had the genre (Epic Fantasy), and a plan — turn a trope on its head. I also had a theme — empowerment. So, I came up with a story based on a trope (chosen one) that was flipped (it’s not THE chosen one, but someone impersonating/faking the ‘proof’) and, based on my theme, a plan: A girl from a remote region loses her family and village due to the choices of the aristocracy. She plots to overthrow the kingdom by impersonating the chosen one (a man who presents to the king with some items and visual cues) and raising a dragon to do her bidding.