It's no secret that the briefcase is the beaten horse of fan theories, and unnecessarily so, considering it is just a MacGuffin and contains whatever the audience wishes it to contain.

Nonetheless, I wanted to toss out my theory.

I believe the Briefcase contains Marsellus Wallace's (MW) collection of Luxury wristwatches.

Time is a running theme throughout the entirety of Pulp Fiction. Each story centers around it, one way or another. Jules' realisation that his time is up, Vincent needing to save Mia from ODing, Cleaning the remains of Marvin in a short amount of time and Butch's gold watch.

The story that features MW the most is the Gold Watch story. Though it doesn't feature the briefcase, I think it provides some insight.
In my opinion, Butch and MW are sort of parallel to each other. I find it fitting that Butch needing to get his gold watch fits with MW sending his men to get his.

Two clues to the briefcase's contents come from the diner. For one Jules says the case contains his Boss' laundry. To me, that means the watches were being serviced and they were cleaned and tuned.

Secondly Pumpkin, upon seeing the contents, recognizes what it is as well as comments it's beautiful. So it has to be something a bit more physical and instantly recognizable than someone's soul...

That could very easily be a fine collection of watches. People would recognize a briefcase full of Rolex's right?

Further, MW is not seen wearing any watch before the Gold Watch story. Ie after the briefcase was returned.

Poolside, when talking to Jules, he is seen without one, not even the shape of one underneath his tight yellow sweater. But after he is hit by Butch's car, as he rises and pulls out his gun, you can briefly see his watch under his right sleeve.

So with all that in mind, I believe the story is something more like this...

MW originally tasked Brett to go to Switzerland with the briefcase but went awol planning to sell the watches, probably in Amsterdam. Vincent was then sent after Brett, and since Vincent is so uncultured hearing the people speak in French he likened the whole experience to being in France. Brett managed to evade him in Amsterdam when Vincent tracked him there. Eventually, Brett returned to LA hoping to broker for his life. MW agrees, but when learning of Brett's location, decided to have them killed and get his collection back.

Vincent returns to America and is picked up at the airport by Jules. Since he just arrived back from Europe, he begins talking about the various difference between America and Europe while driving to the flat. Meaning the beginning of the film is just a few minutes after Vincent comes home, hence their whole conversation.

Upon entering the flat, Jules monologues and asks his French McDonalds question as sort of a way to mock Brett having been hiding in Europe. Brett tries to plead for his life, saying he had the best intentions to fulfill his task and make up for it, but is taken out.

Vince and Jules accidentally kill Marvin, forced to hide out at Jimmie's. When calling MW, he is poolside wearing a tight yellow sweater with no watch, not even a shape underneath his sleeve.

After, Vince and Jules are at the Diner, where pumpkin sees the watches when the case is opened to him, struck with awe at the collection. However, he is spared when Jules gets the drop on him.

Jules and Vincent return to the club and are greeted by Paul who has not seen them in some time. Hence why his greeting is more like a "welcome back home" rather than a standard greeting. MW hugs them both, similarly as if Vincent had come back from a long journey and Jules helped solve this issue.

MW then takes Vincent with him to hunt down Butch. But now with his collection back, he is wearing his pieces again, as seen under his right sleeve after Butch hit him with his car. Ironically, the only reason that happened was because of Butch's own journey to collect his prized wristwatch, which also involved Vincent.

Thoughts?