1) In writing fantasy, the 'sky is the limit'. I mean, know the rules and tropes, then bend them. It makes the story interesting.
2) A lore within a lore. Writing a sort of a 'legend' inside your story will attract reader interest and makes your work immersive.
As for the elements, like the people, the world itself and the environment, try to look for inspirations, then apply your own ideas. For example, in my work, The Human Saint is Bored, the human world, Chersea, is based on medieval European society (as is the common fantasy trope). However, I removed some of the other conventions, like the day-and-night cycle (Chersea has its 'sun' shining round the clock, that they don't even have a clock), and as consequence, they developed a culture distinct from the usual medieval societies of other fantasies.
So how did I do this?
Before I went to write the first chapter of my story, I planned how my world's society will work. The usual aspects: Government, Society and Family, Education, and Other Culture, are already outlined and defined, so that whenever I get confused while in the middle of writing, I have a 'ready' reference. Always keep in mind the 'cause and effect'. Whatever you decide to be their culture, custom and/or tradition will have an effect to how they interact with your MC (who also have his/her own set of beliefs).
Oh and yes, don't forget, the characters actions should have an effect on the world, or else, it won't be believeable. (Like the principle of 'Butterfly Effect'.)
