While I didn't nail the specific tribe, I did predict that the sixth faction to be revealed for Total War: Rome II would have been a Germanic one. The choice didn't fall on the Cherusci as I thought, but on their neighbors, the Suebi, one of the most historically relevant group of Germanic tribes that played a very important role in the fall of the Roman empire. Between them we find, for instance, the Langobardi, that are between the most prominent sources of the culture of northern Italy where I live.
Here's their official description:
The Suebi are an indomitable Germanic culture dwelling to the north east of Gaul. Not a single people, but rather numerous tribes sharing a common language and similar religious beliefs, they frequently raid their Gallic neighbours across the Rhine.
The Suebi, like most Germanic warriors, rely heavily on infantry, mostly lightly armed and armored. The bulk of their troops carries the framea (a small javelin-like spear) and long shields of oval or hexagonal shape. They are also be masters of guerrilla warfare, forest fighting, ambushes and plunder. They also will have special units named Berserkers and Night Hunters.
While trading isn't exactly their best suite, they have a diplomatic edge when dealing with other barbarian tribes, but due to their disdain for outsiders they will encounter sizable resistance from the cultures they will conquer.
With the inclusion of the Suebi, there are only two slots left in the initial (alleged) roster of eight playable factions. Interestingly enough no middle eastern nation has been introduced yet, leaving a considerably large hole to the east of Macedon. We'll have to see if The Creative Assembly will somehow fill that gap with the last two factions, or will instead focus the launch game on Western Europe.