How much of northern Mexico have the Romans taken? I assume they'd want as narrowed a border as possible against a strong state. Although it'll be an expansive initial investment since it's arid, with sufficient aqueducts and windmills the area could be made productive.
They maintain a border line along the Rio Grande, imagine it as a Rhine during Roman times. Extremely fortified with lines of forts with semi-automatic artillery and ballista positions and with messengers ready to bolt at the slightest movement of troops across the river.
Speaking of the Old World, how is Commodus seen? He's the last emperor to travel to Elysium. I remember that there is a statue of him somewhere around the inner territories of Elysium. And if the Old World sees Elysium as mythological as Atlantis, how do they regard Commodus' claims of going there, conquering, and then holding a triumph for the victorious generals and captured native americans? A grand charade meant to make people like him more? Man are all those historians gonna have to take a double serving of humble pie if that happens.
Except for Elysium, Comodo lived IRL. An emperor prone to his own enjoyment without interest in the administrative tasks that his position entailed and who followed the trend throughout his reign of promoting the rise of his favorites, which led to assassination attempts and coups that were totally defeated. effectiveness by people loyal to him or Praetorians. But to Elysium he is seen as a Fearless Father of the Nation.
I need say i was inspired a bit by Draka Domination fanfics when i write that moments.
Some diseases made it over. There was a plague which killed many Elysians and I am such many natives died as well. That was most likely due to an exchange of diseases. I am not sure about about smallpox, bit hygiene is very important in Elysium, so it wouldn't quite as bad.
Elysium applied numerous hygiene and medical projects, but even so that does not prevent them from being immune and a high network of roads and communications including delivery of supplies such as food, also favors a higher rate of spread. The best known example is the Black Death which was spread mostly along Europe's main trade routes when Genoese traders from Crimea fled the Golden Horde and brought plague with them.
The Elysians will most likely not have a true problem with overpopulation for the next few hundred years. They simply have far too much space for that to happen. It would be interesting though, if they reconquered Rome/Italy and settled a million+ Elysians there. Not that I would touch the old world with a stick, if I was an Elysian.
I'll be honest. Elysium tried to conquer America rather than return militarily to Europe. The situation would be comparable to the Black Ships of Japan.
On the other hand, I announce that in the next chapters Elysium will undergo a remarkable provincial change. Mostly to make it easier when making maps... Try to make a good map with rivers and territories...