
The soul idea is one thing, but it's not Bruce Willis' Soul, it's supposed to be Ving Rhames' (Marcellus Wallace's) soul. They say because of the band-aid that he had on the back of his neck. They say the REAL reason for the band-aid was that a production assistant put it on there to cover up a scar that Ving really has on his neck.
Others think it is gold. Could it be Elvis' gold suit, seen worn by Val Kilmer (as Elvis) in True Romance (1993)? Heroin, etc.
Or could it be simply a 20-watt light bulb!?
Really though, it's better that you don't know, that's the whole fucking point. That's why Tarantino did it and that's why he says he never had a plan for it to be anything. He says "It's whatever the viewer wants it to be."
There won't be a sequel as long as it's up to Tarantino. And there shouldn't be. If the studio owns it, give it some years and then they'll fuck it up by making some cheese fest sequel or prequel just like how they fuck up just about every other good movie.
The Vega brothers movie is the best shot at a prequel, but the odds of that movie getting made are slim. Especially cuz Tarantino said recently that he wants his movie to be a smaller Kung Fu movie shot totally in Mandarin.