The oft-discussed construct that ties together the individual pieces of a transmedia property is the storyworld, the universe in which the stories all take place. But I think that's pretty self-evident -- if they don't take place in the same storyworld, they're not connected in the way transmedia needs to be. What's discussed much less, but which is far more important, is tying together the story elements thematically. The only I've heard this much discussed is by Jeff Gomez. From the Transmedia Talk podcast episode 13:
"Before there is any mythology, any bible, any production, the message itself needs to be refined and galvanized. ... so that in any and every iteration of the narrative content, across this complex array of platform, it's got to get infused into it.
Two dudes in robes hitting in each with light sticks may look like Star Wars, but unless there is this notion of balancing what is dark and light within oneself before one can bring that balance to the world around them, unless there is that theme, it's not really Star Wars." --Jeff Gomez
Jeff insists that this kind of thematic unity appears in any property Starlight Runner takes on.
I think this is pretty key. I don't know that I would say a property isn't transmedia unless there's a thematic unity throughout it, but I might say that it isn't as good transmedia as it should be. Without that thread, it becomes just a series of connected stories over multiple media rather than really being able to be seen as a coherent whole.
I'm not saying each piece of a property need be about the same thing -- far from it, that would render the additional stories unnecessary. The through-theme should be broad, and it should be multifaceted. Each story can look at it in a different way, even take a different stance on the theme, form a different opinion, invite discussion and thought. But there should be a broad theme present to unify the pieces, to actually allow for that dialogue between the stories of the property.
I think we have a responsibility to do this, as creators. It takes our work deeper than the surface and contributes to a dialogue. With all the content that's out there, we want an audience to spend time and money on several stories, so they need to be a step above.
In addition, as Jeff points out, transmedia has the potential to inspire a very high level of devotion from an audience, just by its very nature. We must strive to develop strong, positive themes in our work to contribute in a positive way to the human discourse, to the lives of the people we're asking to give us so much of themselves.
To this end, I've started taking a close look at my primary storyworld -- where I have set my novels, a couple in-the-works transmedia projects (and, yes, D&D campaigns) -- to develop a pair of connected and multifaceted themes that I hope will be threads running through everything I set in that world. Further, they are intrinsically connected to the very substance of the storyworld, connecting theme inextricably to actual content. I'll continue developing these as I create, but I hope for them to be a guide whenever I'm contemplating a new story. And I hope other creators will do the same, providing real meaning to their creations and contributing positively to the discourse of art.