Tipa over at West Karana has been posting about Intellectual Properties she thinks could/should be developed as MMORPGs. She hadn't intended to write about Firefly/Serenity. So I decided I would. Let me start by saying this is actually (cross your fingers) an IP in development according to various announcements and discussed in Serenity Found, a book of essays about the show and the universe it depicts. The project has been delayed, but hopefully we'll see it one of these days.
Minor background on the show, because many may not be aware of it. Created by Joss Whedon, the man behind Buffy and Angel, Firefly was a short-lived show about a small crew of smugglers and noble thieves with a universe stacked against them. The broadcast network Fox only aired 12 of the 15 fully produced episodes before canceling the show, but DVD sales and audience enthusiasm inspired the producers to make a movie called Serenity that tied up most of the loose ends. The dialogue was fantastic, the characters brought vividly to life by perfectly-cast, talented actors. A healthy mix of Wild West and gritty Space Drama (can't strictly call it Sci-Fi), Firefly created a world of incredible depth and realism in less than 18 total hours of produced footage. People around the U.S. continue to have Shindigs where they celebrate the wondrous world that Joss created. I HIGHLY recommend that you get a hold of the DVDs (Blu-Ray) and watch this show. "Whew, let go of the megaphone, Rowan."
The 'Verse would make a great RPG environment. The main characters of the show are literally nobodies, trying to stay under the radar of the government. Player characters could be just as low profile, or maybe a little higher. It has always bothered me a bit how many heroes (player characters) there are in . . . Azeroth, for example. It's absurd really. There would be a multitude of environments available as well; just from the produced episodes, there are American frontier-style towns and landscapes, futuristic cities, ante-bellum type societies, Asian-style architectures, space stations, a variety of ship interiors, and interplanetary space, just to name a few.
Factions could include the Alliance vs. Separatists or former "Browncoats," or the Law ("Feds") vs. Criminals/Smugglers. A player wouldn't necessarily have to take sides, though maybe influence/reputation could play a part in what quests/missions are available.
Speaking of quests, many typical RPG quest types would fit in with the Firefly 'Verse: heists, collecting/salvage, milk runs, escorts, defend a location, capture/kill, etc. It would even be interesting to be able to send other players on missions with rewards available upon completion, maybe out of your own currency and discarded gear.
For those inclined, there could be ships to fly, or ship interiors to explore.
Brawler/Operative (Melee (Tank?))
Engineer (Mech Healer/Buffer)
Maybes: (not sure how these could fit)
Preacher (though no magic in the show so no healing)
Companion (would lead to a mature rating)
You could possibly have a major and minor class for each character, like Shootist/Medic, Doctor/Mechanic, Engineer/Thief. The nature of the show means the game could center around a ship and crew controlled by the player a la STO, or it could be more like WoW and others with individuals, either style requiring groups to complete certain missions.
That's my take, partly influenced by a table-topish (or maybe sit-around-the living-room-ish) RPG, based on the show, that I am aware of.