Here's some information about the expansion pack for Doom 3 which will probably come out in Spring 2005.
The story of this add-on will start 2 years after the events in Doom 3. The UAC has succesfully managed to cover-up the disasters at Mars by convincing others that it were a terrible reactor meltdown and it has send marines back to Mars.
In this expansion you play as a combat engineer. Later in the game you'll find a new artifict just like the soul cube. This new weapon works a bit similar as the Soul cube; you need to charge it up by killing monsters. This will allow you to activate Hell Time which slows time but allows you to move and shoot at normal speed.