The original Dark Souls by From Software attempts to blur the line between single-player and multiplayer modes by allowing players to leave markers and messages behind for other players to read, and in turn find markers and messages left by adventurers before them.
Although the game is primarily a single-player experience, the lines are blurred even further by allowing players to join covenants (the game's concept of factions) that allow them to invade other players' worlds. There are a variety of covenants, all of which serve different purposes. Some covenants encourage players to invade and kill innocents, while others task players with taking on the roles of bounty hunters to punish said invaders. For players who don't intend to engage in player-vs-player combat, covenants such as the Warrior of Sunlight and the Way of White allow players to play cooperatively online.
It's an interesting system and one which could stand to benefit from improvements in the upcoming Dark Souls 2. As it is at the moment in Dark Souls, the interface isn't particularly intuitive and new players unfamiliar with the game's many hidden mechanics hardly know what purpose joining a covenant could even serve. There's not even an interface for players to choose whom to group up with, making it difficult—if not impossible—for friends to play the game together.
It goes without saying that co-op and PVP are two huge, and very popular aspects of Dark Souls in spite of how unintuitive they might seem. The desire for players to interact with others online exists, and From Software could do well to serve the needs of players in Dark Souls 2 by implementing better systems for co-op and competitive play.
As an alternative to NG+ (New Game Plus), Dark Souls 2 should offer basic cooperative features from the very start. Much as recent games like Dead Space 3 and Borderlands 2 have proven, players are more than eager to jump right into a cooperative game right from the start. The concept of having two adventurers against the world is almost too awesome for any gamer to pass up—and it would give Dark Souls 2 an additional innovation beyond simply offering more of the same.