I'm only posting here due to the length of my study and involvement with Heathenry of various sorts.
From my understanding the term Asatru is more properly confined to the 'modern' revival in Iceland and groups directly related to the same tradition and interests (ie. more Scandinavian and Norse oriented with regards to tradition, philosophy, and mythology).
I would then place Asatru within the larger scope of Heathen traditions/religions, which would include all the indigenous 'Northen' European traditions and religions of the Germanic and Uralic branches of the Indo-European culture group.
In such I would place any indigenous traditions from the non-Celtic Britons, the Teutons, Goths, Franks, Finns, Sami, etc.
The term 'heathen' just being a modern linguistic convention for taxonomic reasons, to distinguish these from other indigenous and reconstruction based religions (Kemetic, Hellenic, Celtic, etc) and to create a distinction away from the very diverse group related to as Pagan traditions.
These are only my thoughts on the matter after many years of research with such.
I recommend the following links for those interested :
Portal:Heathenism - Wikipedia, the free encyclopedia
Germanic Neopaganism - Wikipedia, the free encyclopedia
Folk religion - Wikipedia, the free encyclopedia
Finnish Neopaganism - Wikipedia, the free encyclopedia
Polytheistic reconstructionism - Wikipedia, the free encyclopedia