I've got that character that I spoke of, the Roman-esque one. (Actually, Nemedian, from the lands of Conan, we are in that campaign world, but same thing). I feel that the "Sword that conquered the world" should be just a bit more than a mere shortsword, but only enough so that an officer would still use a gladius as well as his legionnaires, though the spatha, or longsword did exist as a cavalry weapon. That said, I'm fleshing this out. Have I missed anything?
Gladius: Martial, Light melee
P or S shared focus: shortsword
The gladius is a shortsword, but it is somewhat heavier and wasp-waisted, or leaf-bladed in shape, putting more mass towards the point. This design makes for a weapon that slashes as well as it thrusts, and also the following unique quality:
You can use the Power Attack feat with a gladius sized for you, even though it isn't a one-handed weapon for you. The benefit applies only to a gladius wielded as a primary weapon; A gladius in the secondary hand (or off-hand) is treated normally as a light weapon.*
You can’t wield a gladius in two hands in order to apply 1½ times your Strength bonus to damage, nor gain the 2:1 Power Attack bonus as per a one-handed weapon wielded in two hands.
A gladius is a light weapon in all other normal effects, such as disarm and sunder modifiers, except where Power Attack damage is applicable.
*Special: The feat Oversized Two-Weapon Fighting allows a gladius in the secondary hand the same benefit of Power Attack as a one-handed weapon, even though it is a light weapon already (rather than create some new feat).