Okay, so let's think this through. What exactly was it that created the "Firefly feel"?
- the writing, which means not only the sparkling, insouciant dialogue, but also the characters, the plot twists, and the very disciplined editing. Didn't advance the plot or characters? Out it went.
- The very good-looking but slightly odd actors. Not a single blonde cheerleader or big-chinned Hero among them. For example, the conscious decision to have Jewel Staite put on a few pounds before they started shooting.
- The Alliance, which is itself a writing challenge. They must be implacable, huge, determined, but kinda ineffectual at the end of the day and with a consistant bullying but sincere desire to "do the right thing." They are not Nazis, or the Empire from Star Wars. Many of them truly think that they are doing what's best for everybody and while this makes some of them prissy, it also makes the whole situation much creepier. So Alliance stuff, from things they say to their policies should be self-important, intolerant, and a bit insulting. But not simply Eeee-vil.
- The same goes for the "sympathetic" characters. Inara supported the alliance. Look at Jayne. We never did get the lowdown on Shepard Book; he may have been simply waiting for his moment. Even your friends may rat you out. Everybody has an agenda and you almost certainly don't know what most of it is.
- The mixed grimy/ornamented/bazaar/gleaming tech esthetic. All, mind you, mixed with ten percent Chinese and two percent other non Euro-American. Not only is this difficult to pull off, but it was used, again, in a very disciplined fashion such that each look went with a particular venue.
Miserable weather, old west clothes and setting and grimy 1930's industrial exceptions means outer world. Usually with some bits of gleaming Rollerball white and chrome in the hands of the bad guys.
Beautiful, elegant, but usually just a bit sterile, cluttered and harried means inner worlds. Kinda New Age fantasy tech as run for ten years by Edwardian Brits.
Funky, exposed rivets and cramped, with improvised bits of decoration and Cuisinart/Mac Plus looking appliances means the good guys, whether we're talking Firefly or the brothel, which exchanges rivets for wood beams.
The same complexity and specifity goes for clothes, music, and lighting and each, except for Alliance spaces like the hospital, requires a subtle touch.
There are some other issues about the Firefly universe that deserve some attention. Joss Whedon modeled much of his world on the south and west after the Civil War. If this holds true, then there is probably at least one sizable and possibly growing secret organization of veterans, romantics, and others, earnestly squirilling away arms and talking about how it was only because of flukes that they lost and next time, AND THERE WILL BE A NEXT TIME, they assure you, they'll win. There are probably also browncoat enclaves w-a-y out there.
Now, I'm not saying that these are all good or all bad. But they are all deviations from the big media s.f. norm. OTOH, to me they all seem like they'ld be right at home in a game of Traveller.