Theory: The Elder Dragons are meditations on different philosophies of (or about, or opposed to) nihilism, as identified by their secondary domains. Not a philosophy major, so if anyone knows better than me, please jump in and correct me. Also, I haven’t spent more than an hour or so thinking about it, so maybe I’m way off base with some of this.
Zhaitan - Nietzsche, not surprising that the person people immediately think of when they hear “Nihilism” also gets first crack at the story. Even the name is basically a mashup of Nietzsche and Shaitan. "Shadow" is a deep spiritual darkness that stands in opposition to the blinding light of the Six (and is distinct from the mere absence of that light in fallen gods), and it's no surprise that the Dragon sits on the city of the gods and proclaims “the gods are dead” -- in fact, it’s indicative of authorial intent. Zhaitan represents an assault on higher meaning, in favor of an endless, empty life of mere flesh and blood and endlessly repeated routine (witness the eternal recurrence of the Risen continuing to work as they did in life, without purpose).
Mordremoth - My best guess is Kierkegaard, or rather it represents a sort of anti-existentialism? It’s not exactly solipsistic, but thematically there is a rejection of the distinction between self and others (which works for Mordremoth since the Sylvari minds, both Dream and Nightmare, are all part of the same gestalt as its own mind, and similarly it views its body as the entirety of the jungle it controls and corrupts), and as far as the Mordrem are concerned, there is a suppression of authentic individuality into perfect sameness. The dragon fails to properly conceive of itself as a particular embodied individual consciousness in a separate world populated by more of same. I’m not super sure about this one, but this sort of intuitively works for me.
Kralkatorrik - Buddhism (or Schopenhauer if we’re sticking with Western philosophy) make the connection between desire and suffering which is very much wrapped up in Kralkatorrik’s Fury -- his unbridled will to consume which directly causes his suffering.
Jormag - Heidegger. This Dragon addresses the political side of the discussion -- it corrupts via the Will to Power. Here is a picture of Bangar.
Primordus - Kelvin (he's a natural philosopher). Think about it.
Sclerybda, or Selbbub, or whatever - No idea, obviously. We can never know. Oh, well, there you go. I like the vaguely Lovecraftian notion people have suggested of a dragon that literally corrupts knowledge and memory, even remotely, through poorly understood means. The dragon of bitrot, the monster in the realm of Forms. It’s sort of a really aggressive take on epistemological nihilism that goes beyond philosophy into applied information theory -- the deliberate erasure of knowledge and meaning to create absolute uncertainty.