Meta Example: The development team finished the game and found they had some cash left over in their Marketing budget. Rather than just buying another round of ads or pocketing the money, what did they do? They got the guys from Extra Credits to do a number of special episodes about the Punic Wars!
As with all the TW games, generally mucking about and defying history is awesome. Showing the Romans what-for as Carthage or establishing a prosperous, united and technologically advanced Celtic Empire is pretty much the least of what you can do.
Outwitting the Parthians with your own cavalry and archers, taking the entirety of the British Isles, and successfully taking the fight to Germania; in short, using your own knowledge and prowess to do what the historical Romans could not.
Conquering the world as Sparta, especially since you'll have to face armies who were designed specifically to defeat hoplites, such as Macedonian Pikes.
As in most of the other Total War games, when your cavalry suddenly charges in and attacks the enemy in the rear whilst your infantry holds them in place with a desperate slog. Almost feels like a heroic, last-minute charge, and like in the movies, it can completely decide the battle within a few seconds.
When you command a thousand Oathsworn and GallicNoble Horse and rout three, four, or maybe even five or six thousand Roman legionaries. It shows us just how spectacular the physical prowess of the Gallo-Germans was, and what they could have achieved had they united and pooled their resources to get their best equipment together.
When you hold an unwalled town from over twenty years of attacks from full-size armies with nothing but a fully-upgraded garrison (easiest for Romans and Hellenics, but possible for all).
Destroying armies with nothing but ranged weaponry. Hard, true… but when it's done, you have a very good chance of having 0 casualties.
Any victory you gain without any casualties at all (usually through outnumbering the enemy, having better troops, and better tactics).
When you enter Cinematic Mode for an artillery piece. Basically, you're suddenly in control of that artillery piece, allowing you, who might very well be an expert sharpshooter in First Person Shooters, to bring in meta-skills.
As in previous Total War games, any time you achieve a Heroic Victory by bringing a seemingly hopelessly outmatched force into battle and curb-stomping a supposedly superior force with relatively few casualties.