How prevalent were female military leaders during the reign of the Roman Empire?

Two things to quickly clear up: this is not a new change to the game, it's been there for months. Also, almost all of the game is set during the Roman Republic, not the Roman Empire.

But anyway, onto those percentages. I'll first discuss the barbarians, and maybe go back for the eastern kingdoms if I have time.

For the female military leadership among barbarians in the time of the Roman Republic we have very, very little direct evidence, but some decent indirect evidence. Almost everything we have comes from Tacitus, who was writing in the late first century AD and the 100s AD. As sources on tribal politics goes, he is an excellent source because part of his job was helping Trajan formulate foreign policy. He wrote Germania, an ethnographic survey of the Germanic tribes, in which there is no mention of female military leadership and Tacitus constantly uses male pronouns when discussing leadership. However, it is also implied that the door was open to female military leadership, although the stars would have to align for such a woman. He firstly says that Germanic leaders were generally picked on merit, not by birth. It is also noted that women took part in many military activities such as hunting, and were usually part of the military camp to spur on their husbands, which meant that if things started to go disastrously wrong, some women could get in on the fighting. Tacitus also says that, generally speaking, each 'unit' of a Germanic army was led by the leader of a clan or group of smaller clans. In a confusing section on inheritance laws, he explains that women did not inherit land or power, but also explains that women were fully expected to have their own wealth, which suggests that there was a bit of wiggle room. From this it seems that although a woman leading an army was possible in a Germanic army, it would have been truly rare. You can read the full text here: https://facultystaff.richmond.edu/~wstevens/history331texts/barbarians.html

However, we do have evidence of it from Cassius Dio, and some archeological evidence, that women did sometimes lead. In his campaign against the Marcomanni, Marcus Aurelius found himself facing female warriors, and captured 10 for his Triumph. We have also found what is thought to be the body of one such woman, found at the sight of the battlefield with a sword, spear, some rather good armour (indicative of wealth and/or leadership) and several cuts to the bone indicative of a stab through the ribs. There is also the case of Veleda, who had a cult-like status among the Batavi and devised strategy during their rebellion against the Romans in 69-70 AD. For the Germanic tribes, female military leaders are a bit out of place, but not totally erroneous either.

The Celtic tribes are a very different story. Tacitus' father-in-law was Agricola, the governor of Britannia, and Tacitus displays decent knowledge of the British tribes (which may be why he was asked to investigate the German ones). There is obviously the example of Boudicca, who led soldiers from her chariot, but in a speech Tacitus attributes to Boudicca it is revealed that female military leadership was not unusual:

"It was customary, she knew, with Britons to fight under female leadership; but now she was avenging, not, as a queen of glorious ancestry, her ravished realm and power, but, as a woman of the people, her liberty lost, her body tortured by the lash, the tarnished honour of her daughters. Roman cupidity had progressed so far that not their very persons, not age itself, nor maidenhood, were left unpolluted. Yet Heaven was on the side of their just revenge: one legion, which ventured battle, had perished; the rest were skulking in their camps, or looking around them for a way of escape. They would never face even the din and roar of those many thousands, far less their onslaught and their swords! — If they considered in their own hearts the forces under arms and the motives of the war, on that field they must conquer or fall. This is a woman's resolve; as for men, they may live and be slaves.!"

This is a very weird passage, mainly because Tacitus is clearly still just narrating (he even refers to Boudicca in the third person) but claims that this is what she said. The most notable part is the mention of “fight under female captaincy” - a woman leading troops was normal in Iceni culture. The contrast between Roman patriarchy and Iceni culture is a focus of this speech, particularly in the last line. I suspect Tacitus is taking this idea and running with it a bit too far, since his whole attitude to Britain is that the Romans were enslaving a noble and respectable people in the Celts, so he may have made the contrast between the Romans and the Iceni greater than it was to really press his message. However, Tacitus also doesn’t just make things up without some basis; the comment about having been led by women before seems to be accurate from another of his works, Agricola, in which he remarks “they [the Iceni] make no distinction of sex in their succession”.

The later Roman historian Cassius Dio also wrote that “ all this ruin was brought upon the Romans by a woman, a fact which in itself caused them the greatest shame.” though a later comment (the person who was thought worthy to be their leader and who directed the conduct of the entire war, was Boudicca, a Briton woman of the royal family and possessed of greater intelligence than often belongs to women....) suggests that he might be projecting a little. Regardless of that, it seems from what evidence we have that the Iceni had no problem with being led into battle by women.

The Brigantes, a major tribal confederation in the modern north of England, was also run by a woman at this time, Cartimandua. She had ruled this confederation for over a decade, at least as early as 51 AD, and was overthrown by her ex-husband in 69 AD. Tribal confederations were almost always elective, so her position of power further indicates the Celts had a far more relaxed attitude to female leadership than the Romans. From what we’ve got, 15% for Celtic tribes seems reasonable. It's impossible to know for sure, but Rome II is a game not a textbook so I've no problem with them taking a ballpark estimate.

As for other tribes, women do crop up in leadership roles, but not that often. For example, the Sarmatians had a queen named Amage who led a small raiding party into Scythia in the late 2nd century BC. Such instances are rare and the 15% figure is almost certainly way too high, though it shouldn’t be 0% either.

So for the barbarian tribes, 15% is generally too high except for the Celts where it seems pretty reasonable based on our limited evidence.