Indeed this what I am thinking, he and his successors would need to fight quite alot of wars to gobble up all the tribes, and then several more to keep it all together, because attempts at secessions will be made by local nobility/aristocracy. I do not think it's impossible tho. If he could gain the loyalty from some of the more powerful tribes through bribes, marriage, concessions, whichever works then he and his successors might be able to pull it together. It's gonna be a bloody affair tho, and is gonna need a line of very capable men in charge, but that would not be the first time in history that was needed and happened.How??
I don't think it's impossible for Teutoburg Forrest to be the foundation of an empire, but it would have to take a few generations to actually get the Germanic Tribes to warm up to some kind of unification, actually it would take a few wars. You need Arminus and his successors to capitalize on their victory by waging campaigns against the other tribes to subjugate them.
If they pull it off they are gonna need to spend quite some time integrating the different tribes and break down the tribal societies, i do not think there will be much expansion soon after the initial unification.