One way to approach this theme is to simply look for when the biggest gaps are in the story (such as the gap between Fire and Flight and the Forbidden Grove, or the long wait in KotBW). There can also be big gaps for certain characters. But, yeah, it might require some re-reading! Here are some ideas off the top of my head:
Since the Quest mostly focuses on Wolfriders, there are many more gaps for the other tribes than there are for them.
We don't see anything at all about the founding of Sun Village, we only hear about it. There are lots of changes in the village during the long wait during KotBW. How and why did some villagers decide to become Jackwolf riders? How did the villagers react when Rayek brought those first zwoots to be domesticated? How did they react to seeing "old age" in elves for the first time when Woodlock and his family grew old and died?
Likewise, we know very little about Blue Mountain. Those early years (centuries) of idealism must have been glorious. How did they first bond with the giant hawks? Did they steal eggs from already huge wild birds to domesticate the chicks or did they actually breed these birds to be unnaturally huge, maybe using flesh-shaping magic? At first did most gliders have hawks (like Wolfriders have wolves) or was it always a select few? (I can picture an early Blue Mountain that was secure yet free, with many more elves collecting resources by hawk back, as obviously Voll did at one time, much more like our myths of the under the hills realms of the Fay, which were light-filled and wondrous, not gloomy).
Go-backs and Trolls have a history, too, that we don't really see.
If you have a character who is a particular favorite, just run through their history and see if there are gaps not "on camera."
