[X] You will spend months and months carefully crafting thousands of pages of setting fluff and defining everything in careful detail and creating a tight and detailed metaplot.
[X] You will spend months and months carefully crafting thousands of pages of setting fluff and defining everything in a few nations or a single region in exhaustive detail, while spending five or six pages loosely giving ideas for what the rest of the world is like.
Your setting needs to be the best one you can make, with all the knowledge you've learned since you first tried this. You write endless reams of fluff about this sci-fantasy dungeonpunk cyberopera, of the ringworld Xvorzieat that your main game will take place in and all the other places in the universe that are far less important, because who would want to play anywhere but Xvorzieat?
You also create a metaplot about how the universe is doomed unless chosen heroes can step up to save it. Of course, these chosen heroes can't be the player characters-that would give them far too much influence in the plot. You make up new and interesting characters for them to interact with and help on the quest to save the universe.
You're almost done with the book now! You just need to figure out how to deal with balance issues.
[ ] I don't believe in game balance.
[ ] There is no problem with one guy being a street-level thief and the other guy having an atomic death robot, they can still have fun.
[ ] We'll make random ways to take away player character agency and suggest using that as balance for more powerful stats.
[ ] The powerful stuff is harder to get unless you get lucky at chargen, to balance out how it's more powerful.
[ ] We'll make all player characters start off equally uncool.
[ ] I'll say that the game is
deliberately imbalanced for thematic reasons.