A Romulan ship designer on the subject: "

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I would LOVE to see the D'Deridex be the first Trek ship with decks that don't all share parallel planes. In which case, this ship would be an ideal candidate for something I, personally, very much want to see - spherical lift cars.
What do you think would be some advantages of a nonparallel deck structure like that? This reminds me the ship was originally to be vertically oriented.
Well, for me, as an engineer, I tend to think of the functionality first.
So a ship's general arrangement and construction is going to be driven, not by some "wouldn't it look cool if we did this" as much as it's going to be driven by "we need these bits of hardware, in this orientation to each other, held together by this much structure."
Once you have that, then... and only then... do you start stringing "space condos" into the spaces in between those mechanical systems.
One has to assume that there's a functional reason, at least as much as a stylistic one, for the D'Deridex to be laid out as it is. The "twin clam-shell halves" hull structure isn't exactly conducive to laying in rational deck-plans if you assume every habitable space must be paralle to every other habitable space, is it?
But... what if the "decks" in the twin-clamshells are, rather, arranged so "up" is always normal to the hull surface (or nearly so in any case)? It might seem a bit disorienting in a smaller vessel... walking the length from one nacelle to the next would involve walking over a significant curved region. But in a ship as large as a D'Deridex... you'd barely notice.
It just seems to me that the internal layout of those structures would lend themselves more towards the "normal to surface" deck layout than towards the "everything parallel" deck layout.
FYI, the reason I like the spherical-lift-car concept is that it can reorient itself to whatever location it's arriving at without requiring any special hardware. This doesn't work in a ship like Enterprise (any of them, so far, with the possible exception of the -J) but for the D'Deridex... well, the amount of space you save by going with "normals" deck layout would more than compensate for the slightly larger lift-tube runs.
Anyway, that's MY take...