I feel like the issue with an Avatar mod would be twofold; first would be a need to spin a lot of the mod out of whole cloth, just due to the ironically heavy lack of world building. And the second would be how to make the game outside of the hundred year war interesting, because a game like PC would need a fairly large time frame for gameplay purposes.
It definitely has interesting possibilities, but it is a daunting task.
I think you could go as far back with Avatar Yangchen as the sort of 'classic medieval' period, and Avatar Kuruk being a party boy leading to instability.
I don't know if you could sort of simulate a 'crisis' like in Stellaris, but having a big 'once an age problem' could be a way of helping things stay dynamic, starting with Avatar Kyoshi and Emperor Chen's conquests. Though, her formation of Kyoshi island then would become hard to do, I wonder if it's possible you start with a straight crossing that she can destroy or something like that. Like a reverse canal.
Similarly the Earth Kingdom could be given dynamism by simulating it's rise and fall, where perhaps early game you try centralization, mid-game it decentralizes after the Chen incident, then of course you have to manage the Dai Li, and then that turns into something where the Earth Kingdom operates more as a loose alliance network. With paradox trying to simulate national collapse better this could be better emulated. I also figure that you probably have a 'Four Nations' IO, or rather four IO's for each of the nations. These could help split the nations into smaller tags, but also keep a sort of power bloc situation where they unite in response to invasion. Or fail to if their diplomacy isn't great. And it could also offer paths to diplomatic unification as a way to engage players without needing to blob out entirely.
Another situation you could play with is the Fire-Lord strengthening an autocratic rule in the Fire Nation and their early industrialization (one of their supposed aims for conquest was to spread this revolution outwards). I think logically you could also do something with the discover of Coal in the North and South Pole, canonically they are also home to large oil fields.
You could also keep going into Korra's time period, the question would just be if you could properly simulate some of the steampunk elements as different units. Also hopefully roads could be upgraded into railroads, which canonically exist in the Fire Nation and Earth Kingdom.
I think one of the trickier elements could be properly distinguishing between the earth, fire, water, air, and non-bender military's, some sort of 'Doctrine' mechanic would be very useful for that.
What might be trickier is the advent of air-power that factors into Avatar. I think people would lose their minds if tools for that are somehow included by Paradox. I know in the HOI4 mod they create a sort of early 'radar' with watchtowers and trained hawks, with how line of sight is now a larger mechanic I think you could actually simulate that in Project Caesar. A building, or perhaps an upgrade of some kind, that reveals units from tiles further and further away. Air-Nomad Sky-Bison and Gliders weren't exactly used for warfare though, so maybe you could skimp out on that, but hot-air balloons and zeppelins were a huge part of the plot. IRL war-balloons were innovated on in the late 1700's, but purely for reconnaissance (so there's hope maybe Project Caesar includes some war balloon mechanic), but after that is when you start getting into the real steampunk stuff.