@Ryujin I think Star Wars and Star Trek would like to have a word with you about not being considered current SciFi universes that are multimedia.A multimedia franchise is a production that ties in different mediums, in order to expand upon the basic story. It can use traditional broadcast media, internet streaming, virtual reality, games..... That doesn't really apply to Firefly. The only SciFi show that I can currently think of that this label would apply to, is "Defiance." That was a Science Fiction series that was filmed in Toronto (the sets, down along the lake front, were rather amazing). It was not just a TV series but also a video game and things that happened in the video game were incorporated into the story for TV.
@LuisCarlos17f The name is not the issue here. You could call something Multimedia or Transmedia, they are similar and related concepts as that Wikipedia article indicates.
Maybe I am being unclear in my argument, but you seem to be arguing that in order for Firefly/Serenity to be multimedia/transmedia you have to mash in a whole bunch of other things (necromorphs, corporations, etc.).
But that is not the definition of what multimedia is. Anything can be multimedia.
Firefly/Serenity as a space western can be multimedia (I would agree with @Ryujin and @trappedslider that it is not currently multimedia). In order for it to be multimedia there simply would need to be development to get the characters and setting into something more than the TV/Film niche it is currently in. So, perhaps some computer games, comics (which appears to be on the way as per @Umbran), novels, etc. It can be a space western and become multimedia without adding any other genres to the mix.
Anyways, I am beating a dead horse so I will leave it there.