Let’s say for some reason the editors want a new special edition for a box set of the Chronicles they asked the authors FC Yee and Randy Ribay.

Like for the Appendices to say the Yangchen Novels I could see learning more information about the Plantinum Affair and Earth Kingdom Civil War between Earth King Feishan and General Nong at least for Dawn while Legacy could contain an excerpt from The Secret History of The Order of the White Lotus about the Yangchen Era like their involvement of the platinum affair as well as leaking out secrets about what happened in Bin-Er to Earth King Feishan.

For The Rise of Kyoshi a friend of mine had suggested that the appendices for the Kyoshi's  could include some of Jianzhu's exchanges with powerful people just to better understand how his net was weaved and how it shaped events. While for Shadow it would include Kuruk notes where he touched upon each dark spirits include other spirits from the original series and Korra the ones that we know in the books. Also What style of writing for these notes be like at least in-universe. There would be sketches all of the dark spirits similar to Diablo’s book of Cain where Most of the art is tarnished, and stained. Speaking about spirits It would be cool to have a new information about him besides the description like say notes that Glowworm is one of the very first spirits or Primordial spirits along with mother of faces and then he notes that Koh and the Spirits of ocean and moon are part of the second or third generations of spirits considering the former was old enough to witnessed them crossing both the spirit and mortal world since the very beginning. Regardless this new information probably gives us a hierarchy or division within the spirits that we don’t know in the lore. We know thinks to Beginnings that Kuruk knows about Raava so he have to have an entry on her and Vaatu as well.

I also think the appends would’ve included background information on The Camellia Peony War and the Reign of Fire Lord Chaeryu.

For the Reckoning of Roku I could see it’s appendices includes the excerpts of The Lives of the Avatar by Jinpa from Szeto to Kyoshi or just Kyoshi entry so that we could learn what other events she was doing during her long life, even if Jinpa may have not lived to see all of it.

But it also will include a family trees mainly for Roku, Yasu, and Ta Min so that we could learn not only their clan names but also the names of their parents and siblings (in the case of Ta Min.) it would also clarify which side of the family tree Roku & Yasu grandparents are on. Are they either the maternal or paternal side?

It would also include a king list/Fire Lord List or a overview on the history of the Fire Nation from the end of Camellia Peony War in 295 BG to The Announcement of Avatar Roku in 66 BG basically covering a time period of 229 years as we could get to learn more about the rest of Fire Lord Zoryu his successors leading into Taiso reign what policies they made during that time period and how impacted the Fire Nation?

Finally while it may be a bit controversial since I know some people like mystery, but in this case, it would make sense to tell the History of Lambak Island since we don’t know what’s true and what’s not given the fact that Chief Ulo is a liar and if you wanted to, you could have it this section being written by a Fire Nation Scribe or at least someone being part of Sozin and Dalisay entourage on the island, basically speaking and talking to the locals, and then preserving their oral history by writing it down, basically reconstructing what their actual history may look like even if it may be inaccurate, but it’s probably the closest think of this similar to Diego Duran attempt on Aztec history for context Diego Durán was a Dominican friar best known for his authorship of one of the earliest Western books on the history and culture of the Aztecs, The History of the Indies of New Spain, a book that was much criticised in his lifetime for helping the "heathen" maintain their culture.

Another idea maybe the History of the Lambak Island section could be written by the natives themselves something more akin to El primer nueva corónica y buen gobierno(English: The First New Chronicle and Good Government) by Felipe Guaman Poma de Ayala.

Regardless, most of the history, will it cover will be mostly from the island early history to Chief Ulo’s death and to play with the universe aspect of it more maybe you could have it be written with Sozin propaganda by calling Ulo a separate his chieftain we know Sozin did just that in the epilogue ultimately it would be nice to see a biased account of the events of Lambak Island conflict through the propaganda?

Other wise what would be your Any ideas for what would be the appendices (at least similar in writing to the return of the king's appendices.) to the Yangchen, Kyoshi, and Roku Duologies?