Volition says the next downloadable content for Red Faction: Guerrilla will be available on September 17th. Contrary to what I said, it seems like it will focus solely on multiplayer, adding eight new maps and a new multiplayer mode called Bagman (also playable as Team Bagman). From Volition's community site:

For those that enjoyed Red Faction 2, you may recognize the mode names above, as we've brought them back. Your objective in both modes is to grab or protect the bag, and then retain possession for as long as possible. Both new modes can be played on all of the non-DLC maps as well, which can make for some very interesting matches. If you ever wanted an action-filled match, try Bagman with more than 10 people on a small map.