My personal theory is Sharlayan is one of cities of the Ancients. The side quests of Amarout mention a sister city. Numerous Sharlayan ruins near Idyllshire and Sharlayan type dungeons name things after Greek routed words. But the dungeons surrounding Amoarout also have Greek routed names. I think the ground work has been setup for Sharalayan to be the Ancients that followed and summoned Hydaelyn. Their isolationist tendency is probably to hide this history from the rest of the world. And makes a pretty exciting turn of plot when all of the scions are either from Sharlayan or have a Sharlayan education.
I think the untied up plot thread of Shadowbringers revolves around Venat, the Ancient who lead Hydaelyn fraction of the Ancients. Notice in the scene following Anamnesis Anyder, he is speaking to 12 others which could easily be recognized as Hydaelyn's Ancient Convocation--furthermore, they claim the act of summoning Hydaelyn is a permenant solution and Zodiark's summoning will just extend the time before the disastrous end (assuming here this is sound). They also say they have had trouble recruiting the "defector" who I'm guessing is us as Azem. Venat does set about sacrificing himself to be the heart of Hydaelyn because he feels he is the ideal candidate, but does say that he won't be gone from the world and his form will be his choice. Given what we've been shown regarding Elidibus, we can guess that Venat is probably a primal and still potentially alive.
Then given that Hydaelyn was successful in sundering the world and sealing Zodiark, putting the Ascians in a bad situation, its not a stretch for the winning Ancients to go on living peacefully. Furthermore, (and I couldn't find the source of this interview) there was an interview with Yoshi-P and one of the writers shortly following Shadowbringers where they were asked what the Ancients looked like under their robes and it was said that its a significant lore reason and can't be revealed at this point. I'm assuming that because the Ancients look like the races of Final Fantasy 14, who are also (partly) the citizens of Sharlayan. Finally, the 12 gods of humaniod races are most likey the 12 remaining members of Hydaelyn's Ancient Convocation.
And if we want to go deeper here (because at this point its pure speculation), I'm going out on a limb and say that Louisoix was Venat. Although this would require a slight retcon of the Phoenix scene from coils, all it will be is a reinterpretation that Venat the primal/Louisoix who summons his old friends (the 12) to empower him which is already the Phoenix to defeat Bahamut (instead of it happening in reverse which is the current interpretation--the 12 made him into a primal). The lore surrounding our character and Battle of Carteneau is we were in 1.0 and Louisoix sent us to the beginning of ARR before becoming Phoenix (which is pretty heavy handed when you realize that you wake up on a ship/carriage that is also carrying the twins who are the direct inheritors of Lousioix's legacy) but we just don't remember the past (regardless if you are legacy or not, the 1.0 players only get a few dialogue nods from npcs). Only Venat, the heart of Hydaelyn would have the knowledge of who we are to recognize saving us over anyone else on the battlefield. While Emet had children that eventually gave us Emperor Varis and Zenos, it stands to reason Venat can have the twin's father and then the twins. The only fault I find with this theory is this means maybe the Twin's destiny will have them preform the sealing spell with Tupsimati (and has been missing since 3.5), which have managed to kill a primal Phoenix and Papalymo (supposedly the best student of Louisoix), seal something stronger than Bahamut or Shinryu/Nidhogg--frankly Zodiark.
Sorry, longer than expected. Tl;dr: Sharlayan is an Ancient's city on the source that followed Hydaelyn. Venat the leader of the Hydaelyn faction is like Elidibus and could be a primal and still alive. Further speculation says Venat was Louisoix, making the Twins more important previously realized.