What has the greatest chance of being in Marcellus Wallace's briefcase?

I've always assumed (despite Avary's explanation) that it was a clever movie reference to the McGuffins in two previous movies.

The most recent being the great Repo Man (Dir Alex Cox, 1984) where a briefcase purported to contain alien remains and which glows mysteriously is a key thread. This, in turn, may have some relationship with the case of radioactive material in the great noir thriller Kiss Me Deadly (Dir Robert Aldrich, 1955).

If you are going to include references to other movies (a known Tarantino trait) those are two awesome movies to reference.