Even before a massive boom of marketing and transmedia storytelling I've been involved in such a thing on the internet for ages. I thought it was incredibly creative how Elegy for a Dead World created an incredibly interactive game where people could just write their own story and make up their own adventure. The Lizzie Bennett diaries were superbly creative and innovative in how they expanded the Jane Austen story and told it to modern times with a girl vlogging. Even going as far as to create twitter accounts for each of the characters where they communicate with one another. I wonder in terms of transmedia storytelling if there's even been instances when people had tried to create fan-created stories based of an original and the original artist/author did not want the world they created to be expanded by fans. I originally did not think of fanfiction as transmedia storytelling but after reading through the articles I now agree that it is because it still follows much of the same rules as the canon. If the marketing piece doesn't relate and follow the same rules as the canon and the stories used exist in a whole other reality than the canon completely I don't think it qualifies as transmedia. Just relation to the canon I think is enough to tell it apart. Transmedia storytelling definitely existed before the internet. People wrote fanfiction way before the internet ever existed and it was definitely possible to market stories through other media like newspaper and television.
For my transmedia project I will be creating either a visual or a auditorial presentation that correlates to a post of my choice from the collective writing website: The SCP Foundation . The website already has transmedia storytelling revolving around it with a couple of games developed from post on the website.