Myths and Mavericks, due out in September, will be a multiplayer DLC for fans of the Red Dead series. It's been promised as free content, mostly as a thank you to supporters of the game. It'll have new characters and new levels for the various multiplayer campaigns.
In addition, the original settings for the Undead Overrun mode (from the Undead Nightmare Expansion) will be brought back in a patch to the game. The exact release date hasn't been announced yet, but for multiplayer users it should be a nice addition to either their 360 or PS3 version.