Gamespot posted the announcing of the expansion Heroes IV: The Gathering Storm. Here is a clip from it.

The expansion, which is scheduled for release in the third quarter of 2002, will feature more than 20 new single-player maps, six new campaigns, and four new creatures. In addition, it will contain 18 new artifacts, new campaign editor features, new heroes, and a new music track. The expansion will also include the Heroes of Might and Magic IV multiplayer update.