I visited my bro at Purdue when I was younger and read a posting on a dorm wall about the theory of the briefcase...marsellus sells his soul (removed from the back of the neck), sends his prophet to recover, code is 666, etc.
Quentin said this, along with who shot who in reservoir dogs, is the question everyone asks.
I do think it would be cool for another artist like rainman to interpret what he believes was in the briefcase and create it. the people who buy the full sets would be the only ones to know. might cheapen the mystery around the whole thing but still would be the ultimate 'secret item' ever created.
just a thought