Arcadia
noun
an image or idea of life in the countryside that is believed to be perfect
etymology
1. A regional unit of Greece in the central and southeastern Peloponnese. Tripoli is the capital and main city with a population over 47,000.
2. A mountainous region of Ancient Greece, named for Greek mythological hero Arcas; legendary home of the rustic god of nature, Pan.
3. Million places in america, named for 1. and 2.
4. Somewhere in Tucumán Argentina.

Arcadian
adjective
1. relating to or constituting an ideal rural paradise.
"the region was not quite the Arcadian idyll he had depicted"
noun
1. an idealized country dweller.
"the dark satanic mills inspired new generations of Arcadians from Blake to William Morris"


2. a native of Arcadia in Greece.

etymology: late 16th century: from Latin Arcadius, from Greek Arkadia.


It looks like a misspelling/variation on a Greek derived word. If it is meant to convey Greek, then it could be to do with the ancients given the naming convention used for them. Allag also uses greek names - scylla, amon etc and is typically portrayed more technologically than the ancients. The three headed icon could be a reference to cerberus (greek, but also world of darkness) as the heads look vaguely canine. The circle effect on some of the billboards is reminiscent of the control panels in azys lla - if you go to helix then up the ramp toward the computer area, the terminal on the left as you enter has a similar pattern flickering in and out over the blue ball in the control panel. This may just be the way projected screens are portrayed by the artist though and not necessarily relevant. As an addendum, outside of the fandaniel echo flashback in tower of zot, I don't remember any allagan technology that doesn't predate the fourth umbral calamity, so well maintained allagan cities may look different than we are used to. There isn't really much information to go off though and I don't know if allag went over to the new world in any capacity. Maybe they're going for a Rome/Constantinople style thing, but roman imagery is usually reserved for the garleans so probably not.

Unless it's sarcastically titled the likely meaning of the word doesn't really align with what we know about the ancients. Unless they want to really lean into the outer wilds spoiler
dreamworld/vr/matrix angle. Echoes of the eye has similar themes of a destroyed original world and retreating into a shared dream. This could be an interesting avenue to explore.
There's also the possibility that it's not one of the convocation's glyphs, but someone of similar status from far away lands. We know there were other cities because the amaurot npcs mention how it's sad that they got exploded or whatever. Presumably they had someone nominally in charge there, even if they were subservient to the amaurot society. There might be a musical motif shared with the solution 9 music and amaurot, but I lack musical talent. Finally the soul suppository translated text could be to do with the ancient's inability to create souls intentionally, could be placeholder or could be some as yet unknown group. It could also be similar to the resonant process - the floor mural in castrum albania looks pretty stylised.

I'm bored of speculating again and will probably forget I posted this in 5 minutes.