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Warning: this answer links repeatedly to TV Tropes. Don't let this happen to you. Since all three methods you mentioned have drawbacks, I'll try to suggest tweaks to them. Let's start with readin...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/44856 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
Warning: this answer links repeatedly to _TV Tropes_. Don't let [this](https://xkcd.com/609/) happen to you. Since all three methods you mentioned have drawbacks, I'll try to suggest tweaks to them. Let's start with reading the genre, then borrowing what you like. This amasses facts of the form "when it comes to X, story Y does it in way Z". Apart from your concern that this is boring or unoriginal, reading the works themselves to get these facts can be very inefficient. Now, writers should read widely, of course; but this is not, IMHO, one of the reasons to do it (at least, not if by "read" you mean "read works" rather than "read about the works", an underappreciated item in the writer's toolkit). Instead, go through [these](https://tvtropes.org/pmwiki/pmwiki.php/Main/FantasyTropes) pages' examples _regardless of medium and genre_, Ctrl+Fing for [these](https://tvtropes.org/pmwiki/pmwiki.php/Main/PlayingWithATrope) keywords; and rather than borrowing these ideas wholesale, think about what assumptions you've made they challenge, then see what that helps you come up with. That'll hopefully make it much more original, much more interesting. The best litmus test you'll have access to is whether it interests you. I contest that real-world settings have to be boring or unoriginal, but it depends on which ones you pick. Guess what: we all (think we) know ancient Rome. There have been so many cultures in history, even in one region over time. The world's [empires](https://en.wikipedia.org/wiki/List_of_empires) alone will give you all the fodder you need for something unlike anything you've ever read. So will the world's religions. I know I keep mentioning _Coco_, but I learned so much from that film about both certain afterlife beliefs and real Mexican culture. Having said all that, **one** culture may not be enough for you. You're trying to write a story that makes internal sense, and cultures often don't. Greek myths, for example, are incredibly inconsistent. (Usually if I look up one of their deities' parents, I discover sources differ.) Cut and change what doesn't work, and borrow from a second culture, real or imaginary, if you have to. For the third option, work smarter, not harder. If your creative focus is on characters first, and you reveal only what you have to about their culture for their decisions and misadventures to make sense, you'll have your best chance of having a first draft that makes sense. Now, let me be clear: you can come up with a Hemingway-style iceberg early on, maybe even spend the first few pages fleshing it out. **You will excise it** , just as you'll excise the occasional inconsistency you encounter in a read-through. Nine times out of ten, you'll only need to delete or rewrite one sentence or paragraph to fix it anyway. Even fantasy is, to some extent, [like realty unless noted](https://tvtropes.org/pmwiki/pmwiki.php/Main/SlidingScaleOfLikeRealityUnlessNoted). Sometimes you'll realise the people in your world couldn't do A the way we do because of B; on other occasions, you'll realise they would in fact do C like us, because people aren't (too) stupid or evil (or smart or good, depending on the parallel). Above all, think about the tech and values of your culture. Have they legalised same-sex marriage, and did they read about the ruling on their smartphones? Feel free to move forward or backward relative to our timeline, of course. Fix what bugs you about the conventions of the genre.