The author goes through all sorts of possible approaches, from a Star Wars Galaxies-inspired ground game to an EVE Online-inspired space game -- even a mostly-social, barely-RPG, casual-type experience. We suspect the last of those is the most likely, if the game gets made at all. As the article notes, the project is not likely to have much funding, and while the audience is passionate, it's also small. That was FOX's problem, too. As we know, the challenges don't end there.
One concern MMORPG didn't bring up at all; Firefly was a character-driven series. Its setting was kind of interesting, but the characters were what made it all worthwhile. If Mal, Jayne, Inara, and Kaylee aren't a part of it, will anyone want to tune in? Turning this franchise into a successful MMO seems almost impossible. Regardless, Captain Mal Reynolds said "we have done the impossible, and that makes us mighty." If the folks at Multiverse pull off a Firefly MMO, that quote could be their mantra.