The PC version of Resident Evil 6 will include a new mode, sort of. The Mercenaries: No Mercy (unofficial name: "The Naries") is based on the existing Mercenaries mode, but with "more enemies on screen than ever before." Ada Wong's campaign will be available when you start the game, playable in co-op, and you will be able to "fine tune the in-game camera," features that were added to the console versions in updates.
Sometime next month, in advance of the March 22 launch of RE6 for PC, Capcom will release a free benchmark test. We'll warn you: regardless of how well your computer handles it, it's still going to be Resident Evil 6.
In addition to the PC news, Capcom also revealed a March release window for Siege Mode on PS3 and Xbox 360, a $4 add-on for two to six people to play online. One team attempts to kill a rookie (AI-controlled) BSAA agent as monsters, while another team controls the living human characters from RE6 as they try to protect him.