One assumes that there has to be an original story in here -- preferably, one that makes sense, that resolves contradictions. Looking for the original story leads one to the Scandinavian sources. Besides being from a more primitive culture, the power of the dragon story would lead one to think that this is the original tale, which later got bollixed up with all that political stuff about the Burgundians and the Huns.
Well, you would be wrong. Turns out the older story is that of the Burgundians and the Huns. And behind the actual events taken up into the tale stand even older elements from the conflict between the Goths and the Huns. In this story and in several others, the Norse poets took over the elements of tales based in other lands and from other Germanic peoples. One of those elements may even be Odin, who is not Scandinavian in origin.
The Norse poets made Sigurd & Co. at home in the Scandinavian milieu and centered their story on heroism rather than politics. Odin's desire to gather warriors in Valhalla to fight the Giants supersedes the expansion of the Huns. Sigurd's slaying of Fafnir becomes fundamental, with his later adventures among the Niflungs an afterthought. All this made it a better story, but the old skalds couldn't ever work out the contradictions in the tale -- especially that of the "conflict of the queens" (Gudrun and Brynnhild) -- because those contradictions were already in the tradition they were working with.
These tales were taken up by Norse poets early on. The skalds remembered details that would have been forgotten otherwise, such as the mention of the Danpar (the Dnieper River), where the Goths once lived. In the Saga of King Heidrek, they remembered the name Harvathfjöllum, from a time before Harvath- had changed into Carpathian. The North Germanic peoples who hadn't left Scandinavia were apparently in regular contact with the East Germanic peoples who were the first to leave their ancient homeland.
For that matter, given the way Theodoric the Ostrogoth became Dietrich of Bern in German poetry, I'd say that all the Germanic peoples constantly recirculated the biggest stories of the day, and saw the heroes of other tribes as worthy of remembrance among their own peoples. In much the same way, the author of Beowulf remembered the Geats and Danes of a bygone era and made them at home in the Old English milieu.
