Foundational rules of your world - the baseline of what is possible

When you are beginning the creation of your world, how do you decide the core theme or genre for stories to take place? Real world history? Fantasy settings you are familiar with? Do you pull from movies or stories? A sudden spark of inspiration that plants a seed you help grow?

I am thinking of the baseline that you want others to experience as they step into what you have created. Do you want them to have a sense of awe and wonder? Feel the mystery and intrigue? Or a sense of dread and horror?

How much does this theme impact what you create next? The people, the places, are they in line with the core theme or do they oppose it?