This is great.

But there are some flaws in the argument. Why would Brett – this random nerdy-looking Californian guy – have Marsellus’ soul? And how do you keep a soul in a briefcase?

A stronger theory would have to involve something else beautiful and glowing with inner light, which also impels people to seek it and even kill for it, which also provides supernatural protection to those who rightfully seek it, but which could fit in a briefcase and which a nerdy Californian might be uniquely skilled at knowing where to find.

My point is, I think Marsellus Wallace has at least one Silmaril.

I love love love theories like this (hi Darth Jar Jar!). As with all the best one so much of it falls neatly into making sense! Right?

One last issue: Tolkien’s elves are usually depicted as fair-skinned and lithe, whereas Marsellus Wallace is a really big black guy. Does this contradict the theory?

It would, unless Marsellus were under some kind of magical glamor to hide his true appearance. And there’s actually some evidence for this.