interesting map and idea. I had idea to implement this battle to my Punic Wars map...
let´s look at your map.
hm, while this is 1 v 1 map - can you do that each player will start on one side? so one player will be Roman and second player Carthagian? maybe each could start with his camp and some units in field? the next ones will start as neutrals?
use "pictures" for units - so elephants will look as elephants (this is better way then have complex legend, I think).
make difference between units (bombard, one way assault, attack up two regions, autodeploy, lose units...).
make combinations of bonuses if held any types of units, but also negative bonuses for any combination of units.
make difference between attacks - elephants could attack infantry, cavalery could attack elephants.
add commanders to map?
use river as impassable and add there som crossing point?
try to read something about ancient tactics and bring to players real feelings-
I will look more at you map and also I will again look how it was near Trebia. good work, I want to see this playable
Oneyed