The Why So Serious campaign for The Dark Knight was a huge achievement (and I'm not just saying that because it I love a lot of the people behind it). It's garnered awards, huge amounts of press coverage, and incredible fan love. I know I'm a little late to the game on this, but I don't think anybody has yet talked about the most important thing that the campaign did, in my eyes.
Two digressions, first. One: Experiences like Why So Serious have come under criticism because they arguably don't create audiences where none were before. At the end of the day, the people who were really involved in Why So Serious were all people who were going to see the movie anyway, right? It's uncomfortable to admit it in public like this, but... yeah, it's probably true.
Two: The most successful transmedia experiences are the ones where there is space for the player to live in the world. Harry Potter, Star Wars, Lord of the Rings; these are all worlds that are very much bigger than the action on the main stage. And that's what we do in the ARG space; we provide walk-on roles that let people live in our worlds, while not requiring them to step onto the main stage themselves.
That's why the first Twilight book is poorly suited to transmedia; there isn't much of a world there outside of the couple in love. But the subsequent books increase the scope of the world more and more, incorporating group dynamics and government structures that add up to a world bigger than just Bella and Edward and their true, sparkly love.
So why was Why So Serious such a big deal? It's because it took a world that did not have space for an audience to live inside it -- Gotham -- and created canon spaces where players could dwell, for the first time. They became voters and accomplices. It turned a property that was previously not very well suited to a transmedia experience and created one that suddenly is. It's not just Batman and his allies and enemies anymore.
And while the people participating in that world are probably the ones who loved the property before, all of that energy and excitement brings more people in. The person with the joker mask was already going to see the movie, but maybe their roommate wasn't going to, or their cousin, or the person they enthuse about the film to at work or at the coffee shop or on the bus.
I know I started reading Harry Potter because of all of the fan energy around it; that's also why I read Twilight. Giving your audience the freedom and an outlet for their passion for your work leads to them converting peripheral audience members into fans, and people who were never a part of the core audience into peripheral audience members. Participation is the engine that drives fandom, and fandom drives success.
So there you have it, one of the most important keys to making a great transmedia world: Scope. Make it roomy enough for your audience to play in your world. They'll love you for it, and their love brings rewards.