I almost always start with setting! I have to know the world before I know how to populate it. I have a tendency to play with doors - between life and death, human and monster, mundane and magic - and with 'ADSOM,' I knew I wanted to play with the physical doors between worlds.

I think a lot of writers are tempted to add complexity by over-complicating things, but always remember that most natural rules/laws are, at their core, simple. Start simple, and build from there, or you risk getting yourself and your readers tangled.

So many people think that if you're writing fantasy, it means you can just make everything up as you go. Want to add a dragon? Add a dragon! Want some magic? Throw it in. But the thing is, regardless of whether you're dealing with realism or fantasy, every world has rules. Make sure to establish a natural order.

V. E. Schwab Quotes