Everything you think about Game Mastering is probably wrong: Part XII - Remembering what is going on
I have seen more than a few people designing overly complicated settings, campaigns, adventures, and encounters and we've all encountered overly simple versions of the same. Most of the time I see this it is usually because there is an attempt to replicate an envisioned reality, ignoring that reality usually oscillates between boring and unintelligibly complex. As you (the GM) are going to be running this reality for the purpose of a game about making choices it is a better practice to start by wanting to envision a reality that is (mostly )comprehensible.
So what is comprehensible? Miller's Law in Psychology deals with Memory Chunking in that most humans can hold 7 (+/- 2) things in their memory at once. So what is a thing ? For an encounter its participants, terrain, consumable inventory, rival goals, and the like. If I have 12 identically armed bandits, that is one thing . If 6 are archers and 6 are spearmen, that is two things . If there are 6 spearmen fighting 3 swordsmen and 3 archers that is four things . The swordsmen, the spearmen, the archers, and the conflict between the two groups. If the terrain is interesting at all, you are already up to five things. If the PC's are on a mission with limited supplies, their own goal of whether or not to get involved is a sixth thing. This is something I try to build into “Seed Tables” (also called a Zzarchovian Wandering Monster or "Zzandering" table).
This will of course ramp up as you scale up. So, use memory chunking again and break the campaign into chunks. Characters, Encounters, Locations, Adventures, Regions, and the Setting as a whole. Each of the campaign chunks is containerized and what constitutes a thing in each campaign chunk is different.
Move up to a location and you have things such as factions, large monsters, major treasures, puzzles and the like. Minor things like a locked desk drawer with a poison needle trap and 10 gold are not really a thing in this context. The room which contains an orc shaman, the trapped desk, and a bunch of scrolls and books with important investigative clues to the complex as a whole would be a thing . The room as a piece of the whole is a thing . A series of small guard rooms full of goblin thieves with a faction goal would be a thing . The giant spider prowling the halls would be a thing. The rumoured tomb of a wight with a magical sword is another thing. The place is in a swamp that has partially flooded the compound is a fifth thing (the terrain). That site is risking being overfull. If I also added in a rival adventuring band that is six (still fine), and a dragon that roams the swamp hunting stragglers its at seven (pushing it), and then threw in a cult of Gulnor the Frog Demon who are trying to turn the complex into a temple...it's now a big mess for most folks (though, 7 +/-2 means some folk can still handle it).
As a region example you might have a quest to overthrow a corrupt baron. You have the things of dealing with the corrupt baron, three site based adventures (freeing prisoners from his jail, allying with or defeating the forest bandits, maybe a monster he won't deal with that threatens peasants), and you then have room to add in another complication or side quest, perhaps something like an invading army of the undead that prevents you from just obliterating his forces but requires you to keep them intact for that future purpose.
Full fledged settings then step it up a level. Each megadungeon or adventure arc is a thing. An overall theme or goal is a thing, examples being carving your own Kingdom out of the region or maybe being British explorers looking for the Eye of Set. Large scale faction conflicts are a thing , such as an occupation of the region by a foreign power, or a war with the dark lord and his orc hordes. If you have a campaign about interpersonal relationships, each of those relationships often become an additional thing at each level. If there is a bitter blood feud or devotional love between two PC's that becomes an important element at the encounter, session, adventure arc, So when figuring out content for your own game, keep that in mind when creating an Adventure.
As a final note, if you are a fan of red herrings and twists, depending on how much time you want players wasting you may or may not want to consider those things. If it's an established part of game play to sift through red herrings and expect twists, might not be a thing. Much like an M.Night Shyamalan movie, it's expected. If these are unusual parts of your game, you might want to consider them a thing because players will spend a lot of time and attention on them over other elements.