One really random thought, although you've already mentioned it's a homebrew world, are you still using the conventional biographical info from the MM. Or in other words, are chromatic dragons still the baddies and metallic ones good?
If so, it might be an interesting twist if the reason for the antipathy between chromatics and metallics stems from a historic event; a betrayal, possibly on the part of the so-called good guys. And that in this city metallics lived alongside chromatics. Would be interesting if the leader of the evil dragons was in fact an incredibly charismatic metallic dragon who assumes humanoid form to fight - wielding some potent sorcery or magical items.
Discovering the lore and the history of the city could be an interesting diversion for the PCs or the focus of narrative.
Another alternative take, the city can't be raised after all, the whole thing is a ruse orchestrated by the dragon king as he is trapped within the city and needs PCs (possibly due to their goodness/decency/alignment) to free him knowingly or unknowingly and is hoping that involving the dragon cult under false pretences will lure a party of adventurers into his trap. The PCs follow the cultists to the city and discover that the cultists, along with themselves, are for the moment utterly trapped in the labyrinthine city, at the mercy of the wretched dragon king. The PCs might even find unlikely allies in the psychotic cultists.
Anyway, just wanted to toss out some random idea. It sounds like an epic basis for adventuring. Hope they like it.
