You can't have some fortress or cave or wagon wreck pop up every five rooms, or things start to feel crowded. You need the vastness as well as the interesting parts.
Make them temporary, once the first PC finds something, give it a certain amount of time before it degrades based off what it is. It makes little to no sense that some of the wagons at the bottom of the shield wall, for example, have lasted over a hundred years. Thinks break down when they are not maintained....
*Have a fissure open up in an unstable area, and once it is found (assuming staff has some kind of way to know this) it only has 30 or so RL days before the landscape will reclaim it.
*A raider camp that spawns in random places with has roaming NPCs of varying combat abilities. The camp itself is defended (also at varying levels of strength), and has a decent amount of loot to be reclaimed by those who wipe it out (tents/mounts/weapons/coins/plot items). Once destroyed staff can choose to drop another somewhere else.
*Landslides that block off certain pathways that either have to be cleared or replaced, changing they way PCs have to play without NEW STUFF!
*Have a certain flavor of spice start acting differently (foragable grains at first, then processed spice later on after the bad stuff would have started finding it's way into the mix).
*A new poison type.
*A random jewel/stone stars becoming scarce due to over mining. (OMG the obsidian mines are drying up! [the actual mines, not the deposits]) Suddenly obsidian becomes much more valuable. Eventually a noble or templar-led expedition finds a new vein and mine building plot begins).
*A disease starts killing trees in the Grey and the nearby groves, Dasari must find a cure before all is lost!
*[redacted because I want to try to do this with a future pc of my own]
*New resource deposit found in the north or south that the opposing team REALLY REALLY wants.
-Want to see Tuluk thrive? Find a reason to get people (characters, not players) from the rest of the world to come there (with or against the wishes of the powers that be) and Tuluki PCs popping in to either give them hugs or run them off!
Have the world ebb and flow, a single desert elf shouldn't be able to search the entire Pah in one RL day and examine every single "safe" place to hide. Same with the criminals/law in the city. I would like to not have to choose between using my OOC knowledge of the game to find safe places or avoid dangerous ones or making my PC suffer because I don't want to use that info which means I have to purposefully avoid safe places. I'd like to stumble across a new cave/wagon/abandoned building or tent/random cool item dropped in a wilderness room or back alley. I'd also like to fall in a hole I didn't know was there, run into a gith outside of Red Storm or Tuluk, or mantis making a move toward the Pah.
I like the idea of player ran plots, but there are so many beings and forces in the known that aren't controlled by PCs, I don't feel like the rest of the world should remain static just because a PC didn't push the Cycle Start button.
Edit: Changed "players, not characters" to "characters, not players" because that's what I meant.