A lot of fantastic information and visual references in this one. Great find and very helpful.
It's clear to me that the Dominate Romans should have some kind of difference between "Border Troops" and "Royal Troops," perhaps with 2 different barracks. Possibly, Border Troops could be made of mainline classes of soldiers, while the Royal Troops are the counter-classes of troops, or vice versa (thinking like the Atlanteans in AOM). I do something like this already with the Imperial (Principate) romans in DE.
Another thing could be making their territory borders more porous. So, their buildings could possibly having a +10 to 20% greater territory range, but a much less (-50%) territory strength, so that enemy borders push back harder against Roman expansion.
Some kind of bonus to Outposts, like upgrading to Watchtowers could be interesting. They would lose the Army Camp and Siege Walls of the Republican and Imperial/Principate Romans, but by upgrading Outposts to Watchtowers, they can project power and cut off trade routes that way. You could have an additional upgrade, "Fortified Watchtower", that adds a palisade around it, with greater health and hack armor.
And while "Greek Fire" is still a couple hundred years past the the time period, a Greek Fire Ship would be a nice addition to their Naval roster. With the Dominate Romans and Gothic civs, we could introduce Transport Boats that are super cheap, but are low health and have no attack, just a large garrison space.
Wonder could be The Hippodrome.
We can introduce "ruined" versions of a bunch of Republican/Principate era structures and Ancient Greek structures for scenarios, campaigns, and skirmish maps. These can be ruins on the map that you can exploit. A great one would be a toppled Colossus of Rhodes, a "Christianized" ruined Parthenon, etc.
Temples -> Orthodox Churches.