Those big, bloody, decisive battles are still famous 2,000 years later exactly because they were relatively rare. Most battles in the period were much less decisive.
I personally hope that most battles in-game cause relatively low warscore and manpower impacts, but under the right conditions very lopsided victories (like Cannae or Carrhae) or very bloody close-fought battles (Heraclea or Asculum). This would be based on army composition, tactics, and generals' abilities- so, say, Cannae would be the outcome of a high-skill general with experienced and loyal troops going up against a poor commander leading fresh troops who makes a bad tactical choice. A Pyrrhic battle would be the result of two strong armies under good commanders both choosing aggressive tactics. Etc.