Once you learn how to understand and apply historical materialism and break out of capitalist canards like the myth of barter, it becomes much easier to come up with the things that make societies feel evolving, nuanced, and alive: internal struggles, subcultures and countercultures, political movements, economic bases, social mores and customs. That, plus having a variety of real-world examples to draw from to avoid falling into the trap of capitalist realism.
Yeah, this is what I'm talking about. These are all the ingredients for a story that's better than 99% of the fantasy drivel that manages to get published.
I've got an idea kicking around in my head for a take on sirens: their don't lure sailors to their doom with their song, they just have really nice singing voices. But their villages tend to be built in shallows in the way of shipping lanes used by trade companies, and gosh, wouldn't it be convenient if we had a reasonable-sounding justification to clear them out?