It might be interesting, and historical, to allow in some circumstances the "barbarians" to raid across the red scenario border and retreat, where the Romans can't pursue. The Roman would need to fortify and garrison camps and cities, rather than just conquering the raiders' cities.
Also, you could allow the Belgica to randomly spawn a skirmisher from the swamp, and the Germans from the forest, or Helvetii from the mountains. I would imagine most of the population was rural, and you would deal with occasional raiders even if you controlled the cities. The spawn rate might be a function of faction hostility, or morale in the nearest city, or how much impassable terrain they have as spawning grounds.
Please consider a unit skill that increases marching speed, possibly at the expense of food supplies.
Please consider a unit skill that allows a unit a chance to disengage from combat.
Please consider a unit skill that lowers enemy unit morale (a unit's bloody reputation frightens its enemies).
Perhaps the cavalry should not be allowed to construct.