I have some experience writing dark fantasy and space opera plus my recent cyberpunk novel. So, I thought I'd throw together all the little rules I made for myself when writing to share with you guys. Ignore them, follow them, or critique them -- they're only opinion.
1. Being good doesn't make you win and it may make you lose as well as lose badly.
2. Curse with real cuss words because real people don't say feldercard or Sithspit.
3. Villains should be terrifying and commit atrocities. Not guys who wear black while being generically evil.
4. Villains should have a reason for what they do, even if it's "I like gold and/or killing people."
5. Heroes should be flawed, if not actually bad people themselves. It's cool as long as they're
6. Frequent traumatic backstories work well as long as you don't make them silly.
7. The world should be full of corruption where money, power, and position matter. You know, like our world.
8. If a plucky rebellious noblewoman or man bucks social convention, actual terrible consequences will occur.
9. When someone is motivated by HONOR, they probably mean an archaic social value system which is horrifying in some respect.
10. People should die. Everyone living at the end with happy endings is a cop out.
11. Don't cheat and make your enemies orcs or robots. Heroes kill PEOPLE in war.
12. If you do use monsters, make them inscrutable and terrifying.
13. Alternatively, if you do use orcs or robots, make it clear killing them out of hand is a hate crime like killing "Natives" or "Savages" as has been the excuse in our land.
14. Racism is a factor for much of human history. It's not magically solved by saying everyone should get along.
15. Ditto classicism. Uppity nobles, unfortunately, ruled for much of human history despite being morons.
16. Speaking of nobles, rulership by birthright shouldn't mean you're qualified for the position. You may luck out with a Ned Stark but you're just as likely to get Joffrey or something in-between.
17. Advanced technology or magic will not solve social issues but will make it easier to screw over the little guy.
18. Magic should probably be mysterious, horrific, or have some kind of terrible cost.
19. Super technology should probably be available only to the super rich or used to keep those without it down.
20. Killing a single man in a well-described way is more interesting than killing six or twenty guys in a "badass" way.
21. If gods exist in your universe, then they should either be mysterious or dicks. Nothing sucks the momentum out of a story more than Eru or Paladine looking out for you.
22. Characters can have mixes of good and bad qualities which should confuse readers. A really racist classicist soldier might be incredibly fond of children and a devoted friend. A psychopathic blood knight may be YOUR psychopathic blood knight.
23. Romance should be complicated, ugly, and possibly broken. A noblewoman may run off with her mercenary lover then leave a week later after living in his conditions. It makes the world seem more grounded.
24. Don't scrimp on the horror of fantasy staples. If you see a zombie, don't think "it's a zombie" but make it clear it's a horrible shambling parody of humanity that stinks.
25. Speaking of stinking, details like the fact bodies often empty themselves or smell terrible after a time and so on are often overlooked details in regular fantasy.
26. It's often good to draw a comparison between "ideal" and "truth." The Night's Watch wouldn't be nearly so interesting if not for the fact Jon Snow believed they were awesome badasses before finding out the reality.
27. Aliens and monsters should very probably be "alien" rather than funny looking humans unless they're an offshoot of humanity. More like xenomorphs or slugs which are difficult to even comprehend.
28. Conversely, don't make "good" races like the elves unless the lesson is, "The elves welcomed humanity with open arms and were subsequently slaughtered by them."
29. If you use traditional fantasy races, make a twist on them. If elves exist in space, making them Space Nazis who loathe inferior humans is a good idea.
30. Also, take a page from Richard K. Morgan. If you do make elves, no reason not to name them something else like Zorath or Ibnar. Throw out the pointy ears too.
31. Grimdark worlds have sex rather than "just" romance. Characters will have meaningless hook-ups, think about it, and act on lust rather than love all the time.
32. Remember the little guys. While almost all the mains in A Song of Ice and Fire are nobles, Alien benefits for the fact its protagonists are a bunch of space truckers.
33. War is a nasty and brutal thing which consumes the lives of its participants like kindling. If it contains any glory for the protagonists, it should be on the backs of many forgotten souls or a hollow thing.
34. Selfishness is a virtue in grimdark. Those who look out for themselves often end up better than those who would die for a higher ideal.
35. Throw the narrative off the best laid plans whenever possible. If the heroes are going to go on a heroic months long journey to find the Ring of Antioch, cut forward the next chapter to find out the ring wasn't there and they're now bankrupt.
36. Black humor is never a bad idea. A good laugh often is the best defense against a good cry but never have the characters take events LIGHTLY.
37. Reward some measure of ruthlessness. Baelor the Blessed's brother probably murdered the Priest King of Westeros to stop a war of conversion and....well, it made him king and saved thousands of lives.
38. You can make a lot of horrible people into likable protagonists just by giving them something they love, be it their family or companions or just life itself.
39. Disease, filth, and poverty are rampant in the worlds. Even if it's on a space station or in the capital.
40. Unfairness should be a fact of life and often bitterly ironic. There might be a Prince who is wise, intelligent, kind, and destined for greatness except for the fact he's third in line for the throne.
41. Wars should be fought for land, power, or slights as anything "noble." Also, the noble thing may make things worse.
42. Don't be afraid to take traditional story structures and invert them. A Prince may come to rescue Rapunzel, only to find out she's locked up because she turns into a cannibal monster during sex.
43. Understand what, if any, gender roles exist in your univese and how people lash at them for good or ill. Ripley was a space trucker like anyone else, Caitlyn Stark tried to live up to the ideal, Arya hated it, and Cersei may genuinely have been driven mad by them. Don't forget they also apply to men as Lois Bujold made point of in how men of the Barrayar race were enslaved by their violent honor culture as much as the women.
44. Occasionally throw in the genuinely good among the ambiguous and awful. It makes things more tragic when bad things happen to them.
45. If the plucky underdogs are against a vastly superior force. They had better have some crazy ass solution or trick because they're going to lose.
46. Sudden violent deaths or long lingering ones can work wonders for a narrative.
47. Try to capture more than just descriptions of places but how they taste, smell, and make characters feel.
48. Slavery is awful and full of unimaginable abuses--but also something very common throughout history.
49. If you have an oppressed population or race, don't make them nobly suffering victims. Oppression doesn't make you a better person. It just makes you oppressed.
50. Sometimes there's no right answer. You just choose one evil over another and live with the consequences.