I think Firefly (and possibly, by my explanation in a bit, the Buffyverse) could be an excellent premise for a MMO or other extension (ie. novels not involving the characters).

I think a lot of fans of the Whedon-verse overlook the fact that his works provide such extensive mythology/universe/story beyond the characters seen on the screen. I am a HUGE Whedon fan, but I would love to see what else is going on in the Firefly universe and it doesn't necessarily have to tie to one of the main characters of the show. I guess what I am trying to say is that the fans, especially the gamers here, should look beyond Mal, Zoe, Wash, Jayne, ect. and see that there is a huge possibility for creativity within these IPs.