The concept of 'barbarian hordes skirting across Europe' isn't farfetched. The Celts were at one point empire builders. Gaul once had a single king (the Biturix; world king); the collapse internally led to infighting not unlike the civil wars of mediterranean kingdoms and empires. A successfully united Gaul trying to conquer huge portions of land is hardly farfetched; it would just be the Celts trying to reclaim what they percieved as theirs. The 'Briton' faction is a remnant of Gaul in Britain (southern Britain would have been included in the Biturges controlled regions, and would have been considered, by Gauls, part of Gaul). Their amibitions were likely the same (considering they commanded the entire host of Britain south of Brigantes territory when Flavius Julius invaded, they were clearly expansionists). Compare them to some of the Hellenic states. City states and the like. What did they accomplish that made them 'more' than the barbarians in this period? And the barbarians weren't all 'on the fringes' trying to hold off the advance of the civilized world. The Dacians certainly weren't just 'resisting' expansion by civilized nations. They built a kingdom, and were a genuine threat to those around them. The Germans were absolutely not just resisting invasion; it was Germans that descended on Rome out of period, and in period were an expansionistic threat to their surrounding civilizations. To the contrary of your point, the barbarians DIDN'T lose, they won. Hugely. Where is the Roman Empire? And why is most modern law of western origin based just as much on Celto-Saxon legal systems as it is Greek systems? The point is though, barbarians weren't just sidelines to the events of the mediteranean, they had their own politics (one of the main reasons to include the Britons is probably their profound connection to the mainland Gauls of the Aedui, not because of their effect on the med, even though they traded tin with Carthage and the Hellenic nations) and concerns. We don't pick civilizations based on just their interaction with Hellenes and Romans and Carthaginians, but on their interaction with eachother, and to what extent. If a faction, in this period, only interacted with the other barbarian factions, it'd still be a valid faction to consider, since they would be a real, major element in their politics; we selected the most major players in the barbarian world. The world was much larger than the sea, and we want, not just warfare, but appropriate political situations.

This whole thing is not meant as a slight, but simply the unnecessity of such an argument; we would love to include more factions. It'd be great to have every city state. However, we won't ignore major powers that the mediterranean would consider marginal because they weren't in their direct sphere of events. We had considered many powers. Pergamon (one I would personally love to see) was considered, as were a number of barbarian powers. Everything was weighed in terms of accomplishment, and importance in their region of the world. The Britons were major traders, and would've been important in the Celtic world. So were the Goidils. However, the Casses expanded more (Goidils only took the isle of Mann and set up some coastal forts in Britain in this period) and also were important outside of the Celtic world, since they controlled great deals of tin trade. They weren't ancillaries to events, they controlled some. They'll be reflected in an appropriate way too; many trade goods in Britain will ensure that trade relations with Britons would be valuable, as they actually were. It's not something we can imitate with rebels. We did explore using their space for other factions, but all of them barbarians (including, briefly, Galatians, but their contribution was very ancillary in this period; mercenaries and retainers more than anything).