This page on The Infoceptor has posted some information they've discovered while scouring through the data files included with Diablo II which they say is evidence of plans to release an expansion pack for Diablo II. The data describes new weapons, monsters, an acid damage type, as well as two new player classes, a Druid and an Assassin. While they say this is proof that an expansion is planned (going so far as to speculate such an announcement will take place at ECTS), it is also possible that these are bits of data left over from things that were removed from the game design in earlier stages of development (like Quake's vestigial dragon), though the info is fairly detailed, including skill trees for the new classes, and the name of the content, "expansioninput.txt," certainly supports the expansion pack theory.