The biggest thing that's been on my mind is if the technology that the Allagans are renown and feared for might not be theirs.
I say this because those in power of the Allag empire had garbs that don't really befits those with such technological grandeur (Emperor Xande dresses like a 5th century monk, Scylla dresses in what seems to be ancient Indian-inspired dress with only Amon looking like he's the closest to modern dress that appeals more to 14-15th century inspired attire).
I know this seems like a petty point but would something for sustaining one's self, protecting one's self or even if one was living in such a high technological time dress more like us (I mean we the player); no I don't expect v-necks or apple bottom jeans but wear that would reflect the society of that time, especially those in high societal and military points of power like the examples I mentioned above (and let me get out of the way, yes I understand the Crystal Tower is a nice homage to Final Fantasy III and it's something I understand and appreciate).
My other point is that they managed to create forms of technology going as far as data management, holo-translucent-like touchscreen monitors and even an advanced form of AI. We, the players, haven't even reached that level of technological advancement yet and the WATSON super-computer, the latest we have to a base super computer (known on Jeopardy) is a low form of AI at the moment. How can the Allagans not only create computers but ones that can operate well over 5 thousand years?
They also seem to manage advanced forms of aerodynamics as we can see in Azys Lla and aerodynamics aside they managed to create containment cells for powerful creatures, primals and the Warring Triad included, Research and Analysis confinement centers (which again, for the time seems like a HUGE far cry from what could be expected for something 5 thousand years prior the evens from A Realm Reborn), teleportation systems and coils (i.e., known for the Binding Coils of Bahamut).
EDIT: They also managed to create the ancestors of beast tribes and spliced monsters and clones; their own people (that of the result of Biohazards in ARF). The closest those of the times now in A Realm Reborn have to splicing is that of glamours, like the late Inquisitor Guillaime of the quest 'A Heretic Among Us' or true Ishgardians drinking dragon-blood to become dragons.
To conclude this I'll wrap it up with a theory of my own; I don't believe that the Allagans, or their technology, is native to Hydaelyn but extensions from different Final Fantasy universes of their own. Please allow me to explain:
Gilgamesh is known for traveling through the Void to venture through different dimensions to get back to his home in FFV and look for worthy fighters like that of Bartz, a well-known FF fact. He's done this through plenty of games, even spin-offs.
I believe Xande, the same from FFIII, had his and his followers souls transferred here with aid of Zodiark (hearken back to when the Scions and Moenbryda explained how to capture and destroy and Ascian soul back in 2.3 - 4. Regular people 'reincarnate' again through the Void and life is 'reborn' so to speak) and the technology is from that of either SeeD from FFVIII or, more which I'm leaning towards, technology used from FFXIII.
This makes homages not only possible but understandable like Cloud of Darkness; she/it resides in the Void, the self-same void that exists between all the Final Fantasy universes and that of which Gilgamesh uses for his inter-dimensional travels.