Regarding Roman logistics. I was given the idea of having a mule that buffs the movement speed of troops in a small radius, or the work rate.
Possible unique technologies could be roads, so faster movement in your own territory. Then also the roman caligue could be another tech which again buffs troop movement. Not suggesting using all of these as much as having an ultra fast roman army would be hilarious to do.
On a different note, Rome did start military colonies with its retired troops so what about a roman colony that trains veteran troops? Rather than having them trained from a military camp which is nice, but a colony is much more useful at a macro level.