Dead Island shall soon be receiving some DLC entitled Dead Island: Ryder White. Ryder was the primary human antagonist of the game, and this add-on is purported to add a new campaign which will explain his side of the story. Details on whether or not it shall be co-op compatible are yet to be released, though one of the screenshots in the Blistered Thumbs story shows someone holding a spiked bat in the background, which could indicate that this is not a solo experience.
Ryder White will release in America on January 31st for $10/800 MSP. European territories shall receive it a day later.