D&D example: off the top of your head, think of mid- to high-level “sinister civilization” races. Drow, mind flayers, rakshasa, githyanki are easy. Yuan-ti and evil giants come to mind right away. Efreet, demons, devils, sure.
And that’s just the start. Pull your copy of Monster Manual Whatever off the shelf, and you’ll have a half-dozen more…in every book. No matter what your favorite edition is, you have way more sinister civilzations than you can possibly use—even if some of those monsters are “offscreen” and only namechecked.If you’re the designer of a new campaign setting, you probably have a mandate to make all those monsters fit somewhere. You have to figure out what part of the Forgotten Realms or Eberron is home to the dragonborn. You have to leave room for all those sinister civilizations, for the approximately twelveteen elven subraces…and gods forbid you don’t give the gnomes a nation of their own.
That mandate sucks. My favorite published campaign worlds have said, “You know what? There ain’t no _________s in our world.” (Dark Sun is probably the best example of this.)
If you don’t have sahuagin and kuo-toa and troglodytes and lizardmen in the world, the one or two “wet bad guys” you keep can actually have room to breathe. (Possibly through gills, in this case.)For your home campaign, you can subtract even more. Be realistic about how many sessions you’ll have and how many creatures and places the players will meet. (And by “meet” I mean “kill, then loot.”) Reserve a place in your world only for things the players will actually experience (even if they only hear about them). Don’t have eighteen different kinds of goblinesque mooks. Have one kind of goblinesque mook, and invest them with tons of personality and verve. The other seventeen kinds don’t exist, period.
Adding things later is easy. They publish whole books full of that stuff, man, and you can make up even better stuff yourself. But subtracting is easy only at the start. Subtract! Make your campaign lean and mean.