@adt4m,
If the Romans made it across, and thats an IF, they would have either attempted plain trade and left total sovereignty, or would have, frankly to an extent stupidly and to an extent intelligently, attached some tribes. Based off known campaigns, they would have made some "allies" and used them as protectorates, but depending who they faced could have easily won, or had a tough battle. Unlike in Europe and the Middle East+Africa, where tactics of both sides were somewhat known, both sides in this war would take a while to understand each other. If an emperor chose to send legions across the Atlantic, they would face what Europeans faced in their colonizations and what the Romans faced in most campaigns, especially in northern Europe, fear of what would be perceived as totally foreign and horrifying (self sacrifice, cannibalization, etc.), and homesickness. As in Germany and Britain, they would face a great deal of guerrilla warfare, which the Romans in forested areas often had trouble facing, not standard column and file warfare. If Rome was willing to spend the resources and time on frankly, a useless venture, they could win, but supply line stretch and lack of supplies in men and food, and metal, and horses, could be disastrous, though Roman steel could be a deciding factor.