Luther's authority was his intelligence, his scholarship, his knowledge of biblical languages and, not least, his refusal to remain stuck in the rut of eccleisiastical tradition for its own sake. 

Luther didn't "interpret" Scripture: he simply read and understood the written word AS it was written, just as I am reading and understanding your words before me now on my tablet. There's no mystery here. I can read. I am well-educated, and I am considered to have intellectual ability that is well above average. I don't need an "authority" (a magisterium, if you like) to help me make sense of your reply. God (through nature) and nuture have already equipped me for this, just as Luther was equipped. 

As for all those "Protestant theologies knocking around", I hate to burst your bubble, but Roman Catholicism isn't a scriptural or doctrinal monloith either. Your long familiarity with this blog alone ought to have convinced you of that.