I just downloaded the "Fall of the Republic - Rise of the Empire" (FRRE) mod that gives you some heavily historically based campaigns - and some very large battles right off the bat.
Here is the link, and a slice of the Readme:
http://www.twcenter.net/forums/forumdisplay.php?f=46
Take command as Caesar, Crassus, Pompey, Vercingetorix or Orodes in the waning days of the Roman Republic.
Five campaigns are included:
Caesar - 58 BC
Will you conquer Gaul and cross the Rubicon?
Crassus in the East - 53 BC
Will you defeat the Parthians and return to assert yourself as Rome's master?
Pompey - 49 BC
Will you escape to Greece and build an army capable of defeating Caesar?
The Gallic Rebellion - 53 BC
Will you lead a successful revolt to free Gaul from the Roman yoke for all time?
Parthia - 53 BC
Marcus Licinius Crassus is on the verge of invading, and your generals and sons are not to be trusted. Can you defeat Crassus while avoiding internal strife?
The First Triumvirate features unique victory conditions, challenging gameplay, four turns per year, and much more. And unlike many other mods, The First Triumvirate v1.9 uses the -mod: switch functionality provided by RTW. This means that no files are modified or installed that will prevent you from playing regular RTW games.
Barkhorn.