Without the Roman Empire to form the early boundaries of modern nations, and to unite the people within those regions to some extent, the Vikings or Normans, or perhaps some Germanic tribes could easily come to dominate Western Europe, or further afield.
A couple of days ago I went on a rant about how this wouldn't happen, and it basically comes down to population yield. While an invading/migrating Germanic tribe could assimilate/conquer tribes in areas of small populations, this seems incredibly unlikely in areas with very dense and sophisticated populations like Gaul, Spain, and Italy. In fact, most of the Germanic invasions that allowed tribes like the Visigoths, Ostrogoths, Vandals, and Franks to carve out huge swathes of land for themselves was largely a consequence of the Roman Empire, rather than what many people incorrectly assume was prevented by Rome.
Also, there were quite a few boundaries before Rome. Rome didn't invent boundaries, most anthropologists will tell you boundaries have been around about as long as people have been. And in the iron age, Celtic oppida formed a massive band across much of Europe and functioned surprisingly similarly to Medieval feudal systems. Borders will arise given time, and though they may not resemble modern nations, that doesn't mean the floodgates of Germanic tribes will be opened.
However, in a world without Rome, it's worth noting that Germanic tribes will be much more heavily influenced by Celtic culture, as it seems Celtic, particularly Gallic culture was considered the premier culture before the Roman invasions. Evidence of this can be seen in contemporary Germanic chieftains taking Celtic names (Boiorix of the Cimbri, Ariovistus of the Suebi) as well as Gallic goods and material culture being found as far away as the Juteland Peninsula. So, yeah, there's that