As a mostly Adventure mode player I have thought of some things that would make a world more believable or interesting to play on.
I have no idea how much of these idea's are already making into the game with the new upcoming changes, but I see so much potential to this game I just want to add:
"Pre-Worldgen"
Creation myth involving the gods and mythical beasts, mythical people and ancient civilizations
Including - god alliances, banished/forgotten gods, annihilated civilizations from which only ruins remain.
Civilization with tech levels increasing as they go.
Civilizations "evolving" because of civ or worldwide events, such as survivers becoming an effectively new civilization or the splintering of a civilization in new ones.
Civilizations mysteriously dissapearing because of various reasons (fled to another dimension, banished, annihilated, disease)
Civilization merging: forming a new united civ because of some outside threat or age of peace (might already be the case), empires forming through conquering kingdoms.
Worldwide events
Catastrophe's, affecting all live on the world (possibly related to a god or a civilization acting (e.g. Forbidden technology/magic.) - initializing new ages.
More direct questions about history, giving a more detective / hidden history feeling, making history knowledge exclusive to either learned people or old people or people who could have reasonably known about said events. Making older events harder to figure out.
(Tell me about this Town/Hamlet/Fortress or a specific zone, suggestion to NPCs who might know more about requested information).
Prophecies found in books or on old art / told through the generations. Ingame effect if prophecy is carried out. Or a "post worldgen" which affects events to a certain direction making the player act against bad prophecies or towards good ones.
Alternate Dimensions with portals leading there (Alternate worldgen with same seed for landmasses, or a split at a certain point in worldgen generation two simular but quite different worlds). Could also be time travel, removing certain actors in the past might change the future through simulated worldgen when traveling back to current age.
Forgotten Beasts or Mythical creatures showing up later in World Gen (individuals) or earlier individuals going dark and reappearing later for various reasons (such as prophecies).
More groups forming around individuals like the bandit camps or necromaners now, making certain central NPC's more imporant to worldgen and history in general (for example a legendary emperor).
Forming of hero bands in worldgen, legends formed around these individuals and the gear they used, plus their exploits.