In regards to Atlan, this going to be mostly unsorted ramblings, but hopefully I get an idea or two across.
Aesthetics wise, we should either go more Greco-Roman or straight up some alien shit, I feel like the medieval/early modern knight stuff doesn't do the faction justice. Someone suggested Dwemer armor for inspiration and, honestly, that works? Like that's exactly the kind of thing I imagined Atlan immortals wearing? But, you know, more blue-gray.
Lore-wise, as far as I'm concerned, Atlan had always had a clear theme - an insane xenophobic isolationist army with a state, a dark reflection of Atlantis. This still held true as the lore developed further. Both factions are, as of now, defined by their shared traumatic past - the fall of Old Atlantis, but represent two (and a half) ways of dealing with it, alternating in a kind of cycle.
Atlantis held unto hope, picked itself from the rubble, and tried to rebuild what was lost. They mostly succeeded, for a time, once again becoming a great power and even extending its influence to the Surface (their precursor-type interactions with Greece and such kinda imitate their own uplifting by the Titans). This hope eventually turned into an obsession with past and present glories, arrogance and complacency, leading to their decline, the Epimethean reign (the name is rather on the nose) etc. Atlan initially represented a challenge to that decline, but dire circumstances drove them to desperation, and they found strength in that instead: in order to survive Atlan decided to fight fire with fire, they became an embodiment of the injustice and cruelty they suffered and sacrifice their humanity, to the point where they are more similar to the dreaded Hyperboreans of their mythical past than they'd like to admit. This in turn forced Atlantis to shake itself from its stupor, overthrow the Epimethean regime and try to, once again, to recapture the spirit of Old Atlantis.