When looking over the game thus far, the nature of eras has led me to conclude that part of what the game implies, deliberately or not, is a notion of, for lack of better terminology, entropic decay. The world starts at a Mythic Age, and as the creatures of magic die off, it becomes a Legendary, then Heroic Age. If all the magic creatures, including dwarves, start dying off, it becomes a Twilight, Fairy Tale, or "Civilized" (fully mundane) Age.
This, as a whole, is reminiscent of Lord of the Rings, and many other similar mythic stories, including the Narnia series, where the Golden Age fades, the myth dies, and the magic goes away into a more mundane world.
I also notice this in play as an adventurer, where one event I remember well was traveling from hamlet to hamlet, finding completely same-y individuals from the population pool. Even killing them would make no difference, they'd just come back thanks to a bug. Then, I came across a vampire who had set himself up as mayor. He was obvious because he flashed and had legendary skills and hundreds of items and thousands of stories. I could have killed him, but I realized that this vampire was literally the one thing of note in this chunk of the world, and if I killed him, I'd be destroying one of the only halfway interesting things around, like blowing up a landmark. It made me sad to think of it, so I left the vampire be.
With the coming mythic changes, will there be ways in which myths can "decay"? That is, portals to different dimensions shut down forever, schools of magic forever lost, or once mythic and legendary things turning mundane when their "magic energy source" is cut off. (Such as a city on a floating island turning into a mere plateau, or the edge of a discworld turning into merely a chain of really stormy areas of sea on a globe with undiscovered continents.)
For that matter, you mentioned concepts like myths from different creatures/cultures that oppose, and some of which may or may not be true. If there are conflicting myths, will it be possible for us as players to affect whether they are true or not? (To dip into the more Elder Scrolls style mythic, where reality is partly subjective, belief or disbelief can grant or strip away divinity.) If I lead a crusade to wipe out believers of a contradictory myth, can I make that myth untrue, and take away its magical power? Can we, in play, make new myths, or shape existing ones in other ways?