Why wouldn't it be "ich bin Original"? Don't you usually eliminate the article in cases like this? (e.g., "ich bin Lehrer" instead of "ich bin ein Lehrer"?)
You can omit the article in the case if you see that being original is a profession. Otherwise the meaning without the article would be similar like in English without the article "I am original".
That actually sounds more natural in English. We use originals to describe creations, not humans.
my grandpa used to call himself "the original Frankfurter". It was partially for his sake that I wanted to learn German.