It's been speculated that the briefcase had the jewels from Reservoir Dogs. If you remember, there was a character in Reservoir Dogs by the name of Vinnie Vega, which some guess is supposed to be Vincent from Pulp Fiction.
The Marcellus's soul hypothesis comes strictly from the combination code, and the "boss's dirty laundry" commentary. But as has been mentioned, the briefcase is whatever you want it to be.