During a Pax panel last summer, Tabata commented on the nature of Final Fantasy XV's cosmology stating it has a baseline in Japanese myth with the Western influence. I think we should discuss that Japanese baseline when speculating about XV lore. XV is pulling from Shinto tradition, including the tales told in the Kojiki and Nihon Shoki which are ancient texts that illustrate the creation of the universe, Japan, and the Royal Family.
WHAT IS THE NATURE OF THE ARCHAENS/ASTRALS?
Kami (gods/divine occurrences & beings) in Shinto are basically parts of nature itself. There's kami for almost everything. I think the Archaens are analogous to kami as the elements of creation in Eos.
Kami in Shinto are not only metaphysical they are very much a part of the world. Draw spots in XV could some sort of residual from the creation of the world, like how every act of creation of the Kami left new bits. During a demonstration of Titan's summon attack it was mentioned the earth returns to a natural state afterwards. I think they mentioned that is because the Archaens act as protectors of the planet. I think we should understand the summons as literally a part of the planet so their magic returns to it after attacks.
Next, the Kojiki tells of the first three generations of Kami. The first set spring up, create existence, then disappear. (Kinda like the FNC gods) Another set begins to appear in pairs of male/female until we get to Izanami and Izanagi. These two kami are really important to Japan. This pair creates Japan and every act of creation begets more kami. They also have an interesting story, but I'll get to speculating on that later.
We know six gods/Archaeans created Eos. They are probably Titan, Shiva, Ramuh, Bahamut, Leviathan, and...Ifrit(?). The Genesis mural may illustrate creation in different generations too. For example, Titan, Shiva, and Ramuh, (who comprise two of the basic elements in Eos, Lighting and Ice) sit above Bahamut and Leviathan, who enter later. Bahamut and Leviathan also look like siblings or spouses like the Kami in the second generation, namely Izanagi and Izanami.
We've been told the personalities of the Archaens are important to the story. The fact the summons are characters themselves is interesting alone but I think there's a good chance their personalities and relationships could parallel the Kami in the Kojiki. I'm not sure how yet because I don't know all of the stories. The specifics of the Izanami story and maybe even Susano or Tsukiyomi's tales could make it into the game.
One other focus of the Kojiki and Nihon Shoiki is tying Ancient Japanese royalty to divinity.
WHAT IS THE NATURE OF THE LINE OF LUCIS?
The Japanese Royal Family is descended from Kami like the OG Kami - Izanagi and Izanami. The Lucian royalty is descended from the gods of Eos... or at least were blessed, heavily.
There are artifacts that are enshrined in Japan that only the Royal Family have access to, artifacts from the times of the gods. There's a jewel, mirror, and sword. Lucis has a Crystal, Ring, and (?). The keepsakes serve as a validation of authority and maintain a connection to the important Kami, each artifact representing a virtue to be upheld by royalty (valor, wisdom, and benevolence). Through the FF filter, the artifacts have become tools of literal magic power!
(Straight from Wikipedia:) "According to legend, these treasures(royal artifacts) were brought to earth by Ninigi-no-Mikoto, legendary ancestor of the Japanese imperial line, when his grandmother, the sun goddess Amaterasu (daughter of Izanagi), sent him to pacify Japan." (Hmm, sound familiar?)
We see on both the cover of the Cosmogongy book Noctis holds and the Genesis painting, the rise of the Lucian line related to their pacification of some ancient threat and depended, in part, on their gifts from the gods. Following those initial conquests of both Japan and Eos, Ninigi and Lucis were left in a unique position ...at the top. I think that is relevant to some of the conflict driving the story.