So we've seen the recent episode of Agents of S.H.I.E.L.D. "The Writing On The Wall" and the the big revel of the "city" they're now bound on a quest for, but what struck me when we saw the 3D blueprint of the "city" was - it's underground.
Don't expect to see a city floating in space, or laid out anywhere obvious, this is clearly one that Coulson and gang are going to have to dig deep for. Rooting out all the clues we can expect to see plenty of mysterious rise and resolve each step of the way.
In the comicbooks the city might have been Attilan, the mystical ancient alien city and home for the Inhumans, but is it?
From what I saw in the 3D design this is no space-going city but clearly has the shape and pattern of a deeply buried supercity for..?
From the huge airshafts to the surface through the perspectives of levels laid out and connected by rampways this all shows us this "city" is an underground structure. so let's suggest for now that this IS NOT Attilan, but perhaps a half-way base on Earth, somewhere that leads to further discoveries and a gateways to space.
In the comicbooks Attilan is currently nestled on the moon, and I don't see Coulson and gang getting up there anytime soon. So a half-way house that limits the special effects (until they get a Game of Thrones budget boost?) and still introduces many elements of that city and the Inhumans could be hidden somewhere on earth. A base for alien activities, or a long lost ruin full of "tech stuff"?
I wouldn't go for the ruin theme, Marvel are bringing out an Inhuman film and there's plenty of potential to guide the audience to that cast of characters through Agents of S.H.I.E.L.D.
We'll have to wait and see what Coulson can dig up.