The imperials seem to loosely based on the romans and the Stormcloaks on barbarians. With this in mind I would of liked to have seen a greater difference between the two. The imperials should fight in tight disciplined formations with short swords and larger shields while the Stormcloaks should be charging around swinging greatswords and war hammers.

It would have created two distinct approaches to battle and have been more interesting to play though on both sides.

True, but they're not complete roman or barbarian rip-offs, and besides the game isn't designed that way and most battles are so small that it might have ended up looking silly.

Don't they already?

Considering how tiny all the battles in this game are, it would just look dumb. Three Imperials walking in line against four Stormcloaks running randomly about.
What an incredible amazing and epic battle.