I am interested in hearing more about the Point-of-View you had on how combat in this period worked, and how you then used mechanics to mirror that Point-of-View.
I have a similar rule set using the Wars of the Republic rules from Osprey as a base. However, I am still building Historical scenarios for it.
I must emphasise that these are meant as an extremely fast play rule set, so historical detail has been pruned to a degree.
OK - troop types. Where the L. Romans & their Hun or Goth frenemies have differing troop types, movement & combat attributes will differ. eg cataphracts move slowly but resist missile attack.
This also impinges on some unique mechanisms e.g Hunnic horse archers, if charged, can feign flight, fire & then turn to face.
There are some unique weapons eg plumbata & francisca which can be hurled at enemy charging if the unit, so equipped, choses to stand & receive the charge. Some Barbarian infantry are classified as "warband" - faster to charge, greater impact but poor morale if their charge fails.
I should say that by and large, there isn't that much difference between the L. Romans & the Barbarians.....& I think *that* is also a period-specific trait. So both sides infantry can form a shieldwall fo example.
I hope this helps.
donald