Honestly I would not even think about involving the Ethereal Plane into this story... I don't know the details, or what else you have in mind, but from the sound of this summary all I get is that the key story element is planshifting i.e. actively moving around the multiverse (not necessarily all of it). Why do you want to use the Ethereal Plane in this story?
The TL;DR is that there's an artifact hidden in the city's Border Ethereal "counterpart" or "side" which many factions are on searching for (though nobody knows for
certain it's in the Ethereal, or even that it's in the city yet).
My actual answer is more involved...
Here's the former thread.
The Lost City of Ubar (which I'd previously been calling Qaybar until I found the Ubar references in Wolfgang's
AQ4 Secrets of the Lamp) ir primarily inhabited by jann, lesser genies comprised of all the elements. In my story, these jann settled in what grew into the city, and they are much more mercantile and amoral than the nomadic jann of the desert (at least that's how I'm depicting them). One of the traits of jann is that they can have Innate Spellcasting – specifically, the ability to cast
etherealness 1/day. So they can exist on the Border Ethereal for up to 8 hours a day! I wanted their habitation to reflect that (along with other things like having the ability to
create food & water, fly, etc).
I'm really intrigued by what that means for the jann... maybe they are creatures with very active dream lives, or maybe they regard a child's fantasies with far more gravitas than humans, or maybe they forbid the unauthorized casting of illusions, or maybe they are accustomed to navigating in unusual ways & rely on plant life and/or creatures that can see/shift ethereal to block potential ethereal intruders.
I was trying to think of literary examples of a city with another side...maybe the Plateau of Leng from Lovecraft? or whatever that troll market was in Harry Potter?
I'll be honest that I do enjoy taking things that previously were "why on earth would a DM use this?" and making them fun and imminently usable. For example, I did this with the
planar renovation project for Bytopia. But that's not my driving reason. If that was all I had, I wouldn't have gone forward with the idea. My driving reason is that somehow it
feels right for the jann to have a city with a vibrant presence on the Ethereal Plane....something about the mystery, the mists, the ghosts of the past, the dimensions ethereality adds to thieves & repelling them, the thrill of a chase through the bazaar in both the Material and Ethereal Planes...it just
feels right.
The one visual I have for this is the Prince of Persia game from 2008 (see below). In that game, the player uses magical pressure plates to navigate environmental obstacles in magical ways including accessing something akin to the Ethereal Plane.
Here's one small segment of my writing that's relevant to what I'm doing....
[SECTION]
An Ethereal City. Ubar exists as much on its current plane as it does in the Border Ethereal, where the ground level streets are shrouded in thick bluish mists and the minarets glow faintly against a sea of the cosmos. Special guardians and magical pathways known as the Masarat al-Jann are found in the mists. This is the spiritual heart of Ubar, sought after by malevolent forces which assail the city with ether cyclones (coinciding with sandstorms on the Material Plane or elemental storms on the Inner Planes). Jann alchemists and philosophers believe it is in the Border Ethereal where the Seal of Jafar al-Samal was hidden and where lies hope for the transcendence of the jann race into a form equal to other genies.[/SECTION]