Romans
One of the more curious omissions from For Honor, the Roman Empire is perhaps one of the most popular ancient cultures displayed in gaming. This is not only due to the rich history and mythology, but their powerful armies and weapons. Having the Romans enter the fray in For Honor could make for some interesting conflicts since they were before the time of the various factions. Not only do the Romans have a nice variety of weapons such as the Gladius (short sword), spears, and javelins which were used to take out distance targets, but they had fantastic defensive capabilities.
Roman warriors also utilized large shields that would make them an interesting foe on the battlefield thanks to their solid defense. Given that the Roman army was all about working together in combat, this could create some interesting mechanics that reward users for working together. Since For Honor has game modes such as Domination that reward playing alongside your teammates, a Roman faction would make interesting combatants on the battlefield.