If CA makes a Germanic faction it will probably be an important tribe surrounded by minor factions, like they are doing with the Iceni and Arverni. I really don't see why they would try make the Germans a "unified power".
I think the a German tribe will make it in not because of their importance but because of their map location. Without them its likely that Northern Europe would have no other playable faction, and thus not recieve much gameplay.
I'm almost certain Seleucids are the ones who aren't going to make the cut. Sure they were really powerful in 270BC, but the were on the decline. The Parthians were a rising star, which suits Total Wars gameplay mechanics much better.
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