@r1: Your Mythic PC can be anyone at all in history - a relative, a villain, a hero - anyone. An Exarch of Kord...?
The easy way to get your PC down into the cavern is that he drew a Duty stone when Zlatan and Heskrash came by. As to where he is exactly, he can 'tada!' become one of the Dragonborn NPCs already mentioned or be relatively seamlessly integrated into any of the scenes without Dragonborn.
And for the Bonding, that's just fine.
@hafrogman: Wow! Gutwick!

It was so hard to pick between all those entries, they were fun!
Let's add another group to the 'citizens' of Naedur. A labor class that doesn't enjoy the same rights as the other citizens, but are tolerated because of some Oath or other. Dwarves are very serious about their Oaths. Especially the ancient ones. The Goblins! Periodically as they reproduce beyond the levels allowed, they're cast out of the City (maybe even how the Dragh found the City in the first place).
I'm totally game to let you stat Ehrero Khan up.
@garyh: Welcome back! Bring on Resolute... I never did figure out what the Warforged were doing in Naedur exactly. I did have some thoughts that they were put into stasis as part of the City's defense (or maybe caught up inadvertently in the warding magic), and that there are caches of Warforged here and there. Some are "statues" around the City. That would make Resolute's memories an issue, he'd remember Naedur from over 1000 years ago!

Thoughts? Would you prefer an integration more like the Balash, Dragh or Goblins?