Why wouldn't Dacia be included? They're one of the more well known and documented barbarian peoples. Plus they gave the Romans a pretty good run for their money militarily.
If I were to choose 12 factions for Rome 2 (The optimal number), I would go :
-Rome = of course
-Carthage = Rome's dark twin, it's great nemesis, The Gary Oak to Ash. THE Rival.
-Macedon = The most powerful of the ''pure'' Hellenic state.
-Avernii = The most powerful of the Celtic Tribes, Even in a weakened state and constantly back-stabbed gave Rome and it's most Iconic leader a run for it's money.
-Seleucid = The Titan of the east.
-Egypt = The unavoidable Egyptians. Can't make a game in antiquity without them. Plus a nice mix of Hellenic and Pharaoh Culture. The Rival of the Seleucid, Cleopatra...
- Parthia = Still a backwater province, but about to rebel (one can afford a 20 year anachronism at the start of the game) and to become a mighty empire, one that would defeat Rome countless times.
- One powerful Germanic Tribe = Not that much interesting in early years, only managed to be an annoyance to the Gauls. However, in later years some tribes become a major threat to Rome (perhaps to biggest one). Plus, in early years they are your iconic Wild men. They evolve significantly in later years (100 AD and onward).
- Dacians = A big player, especially in the later years. Managed to defeat the Romans and drive them back. Fairly developed, a strong an well armed military. A mix between Hellenic culture, and their own very unique one.
- A Scythian Faction : this culture spread very far. Some of the tribes were fairly powerful. It will also fill these vast wastes in the north, so that it's not just empty and boring territory. A steppe faction with all the challenge tied with it. Could be interesting to play.
- An Indian faction. One of the Western Region of the Mauryan Empires. While much of the Mauryan Empire wouldn't be on the map, this Empire was far from being unified anyway (which explains why i didnt steam roll Asia Minor on east Asia). They still played an important role in Asia minor, and they are pretty exotic.
- Galatian : A fairly weaker faction, a bunch of Gauls lost in Asia Minor. A completely Alien culture compared to all surrounding it. Some kind of Oasis of Celtic Culture in the Middle of a greco-eastern world. There could be a tons of more important factions to choose from, but these guys are too exotic, I would totally play them on first play through for the Lulz of creating a Celtic Empire spanning all Asia Minor.
If there's even MORE than 4 faction DLC, like maybe 8 DLC, the additional factions should be :
- Pontus
- Armenia
- A greek city state : Just for the fun of playing a small coalition trying to survive around all these new big players, that supplanted them. The Aegean sea would also be a fairly interesting place to play in.
You would control small territories, spread out along the different coast, trying to set up a trade Empire. It would be like playing Venise in a Medieval Game.
- Spanish tribe (lusitanian, Iberian or Celt-Iberian) : Pick one of the 3. Each are fairly unique, were also fairly powerfull. They gave Rome a hard time.
Bonus faction
- Iceni : Just to have the extra funding from the UK government. Because of some kind of nationalist law that gives you money if you feature ''british'' in a video Game. Also they would occupy an otherwise empty and isolated place. They would be a good faction for the beginners, as it would be the turtling faction, due to it's unique position. Also chariots are fun.