The same question could be asked about Ireland, Pictland or the eastern part of Europe, but in this case I'm talking about Germania. Please, don't take it as a complain, it's not.
What's the main reason of having the northern Germanic peoples portrayed in the game but not the rest of them? Did the devs considered only some of them were "advanced" or "civilized" (in game terms) to be portrayed as playable tags? Was the research too hard to find accurate enough locations and names of the tribes? Was it simply for gameplay value and for having empty "colonizable" regions? Will these gaps eventually be filled?