Capcom recently announced that a fourth campaign will be included as an unlockable bonus in Resident Evil 6. In the style of Resident Evil 4's Assignment Ada or Separate Ways, the story will follow Ada as she crosses paths with the main cast. The graveyard gameplay footage below would indicate that Ada's levels may place emphasis on classic horror.
Also announced was a new gameplay mode known as Agent Hunt. It appears to be a zombies vs. humans mode, with emphasis on group tactics to take down the game's heroes. Over time, players can mutate and upgrade their zombie character, providing a variety of tactical approaches in order to gain the upper hand.