My guess is that the current king (or whatever) has foreseen some strange portent of something to come. He knows that WL has some qualities to be a leader but needs to grow the fuck up. Going with the anime stereotypes, we're going to have her as a big, brash confident person who is secretly terrified and insecure about themselves (we've already had a few glimmers of this.)
The city of gold is interesting. It's clear that this expansion has some significant Meso-American influences, although Yoshi P and the narrative team seem to have a raging hard-on for things relating to Greek mythology. A fun twist would be to have the golden city be cursed (a la Midas), although I'm guessing that the more obvious candidate of it being related to El Dorado is more likely.
There's some pretty interesting parallels that SE can take from real world mythology that could apply to both the golden city and the boss encounter we know about (Valigarmanda, or Tritoch since it's easier to type.)
Gold or golden energy, as it relates to the Muisca peoples (present day Colombia), is seen as the creative energy that surrounds everything. Much like aether. Also, t
heir main god was technically a trinity (Sun, Moon, and the Rainbow) and Tritoch is also seen as dealing with a trinity, as the Esper in FF6 taught the primary elemental magics and had an attack called Tri-Disaster. I could see some worldbuilding revolving around this. Maybe with them doing a cop-out ending and having a triumvirate rule things instead of a singular ruler (which would honestly be on par with the general theme of FFXIV's 'can't we all just get along?????' and avoiding conflict at every turn.)
The golden city could also play with some myth making, as it's taken on different meanings (was it a person/ruler? A ritual? A city? A kingdom? Etc.) Maybe it's a symbolic city of gold, either relating to the energy I was talking about or something dumber ('the city of gold was inside you all along!')
There's also the obvious sun parallels and I imagine we'll be seeing some of that. Maybe also the Inca folk-hero Inkarri.
Also curious about the Mamool Ja with the hood over his face. Not sure if the eyes have some sort of 'third eye/fourth eye' symbolism rooted in mysticism.