Okay, Sinister, so if I sing a folk song in my local opera house, that makes it an operatic aria?

That door swings both ways.

As I said, an Operatic Aria becomes a Folk Song in a folk context. So if any song can become a Folk Song, what you should be asking is, can any song become an Operatic Aria if sung in an Operatic context? To which the answer is, undoubtedly, yes, with some considerable evidence.

Otherwise:

As for what makes a Traditional Song, the lines are clearer with respect of a canon of material collected, recorded, catalogued, cut and dried, sourced and analysed, numbered, indexed, with occasions, performers and variations duly noted. Sometimes Traditional Songs might be sung as Folk Songs, other times they might be sung as Classical Songs, Rock Songs, Wyrd-Folk Songs, Jazz Songs, or Pop Songs. Anything is possible with a Traditional Song - but the song, essentially, remains the same, whatever the context.