I envisioned calvary working like this.
Attack at a 2
Move at a 2
Defend at a 3 (If on a territory containing defending infantry)
Defend at a 2 otherwise.
Cavalry played a minor role during the entire war. (Lawrence of Arabia for example)
Cavalry would have one other feature. They alone would be able to leave a contested zone and attack an empty adjacent enemy controlled territory.
By turn 4 they would no longer allowed to be built.
I do agree the game is great as is.