The choice of which alternative faction to add seems to me to depend on which factions people play- personally, I want more action and more detail in the part of the map I'm playing. I mostly play as the Romans or Greeks so I would probably be looking at Syracuse or a Balkan faction. If I was playing as the Sweboz, I might want to have a rival for control of a unified Germanic nation. To me, as long as any added faction can be supported from an historical point of view, it's a gameplay question not absolutely an historical accuracy question (I'll probably get tarred and feathered for suggesting that on this forum). So is the question to ask: What factions do most people choose for their campaigns? There may be a case for leaving out a more "historically significant" faction in place of a less important faction that would have a greater effect on a player. At the end of the day, this is still a game and should be played for enjoyment.