The ability to build tribes and communities with players. I think this would be really good for social interactivity, since a player has a role in their tribe and relies on others for different things. This could help create bonds between players, and overall lead to more fulfilling gameplay. This 'bond' does happen in some multiplayer games, but it isn't streamlined, so it's up to the individual to reach out to other players. There should be a system in place that creates the right environment for cooperation, trust and responsibility for each other, which then makes it easier to develop this bond over gameplay.
I haven't thought too much about this (actually I've been trying to write this post for 2 days but I suck at wording and I'm meant to be doing school work), and it probably wouldn't make a great game by itself, but building a game with this as a core concept could have great potential. What do you guys think?