I think Alan Tudyk has a great idea about how to bring it back. The BIG question is how far forward is it set? Say 10-12 years? 15 or more? I think that might be pushing it. What is the thing that sent Mal "into hiding?" Was it Alliance attention as backlash to the Miranda reveal in Serenity? What would Mal look like - is he still soldierly and clean shaven? What kind of jobs still have him aiming to misbehave, but really small time compared to the earlier episodes? A good joke would be to have him being known as the guy to go to if you love wobblie headed geisha dolls. And since it's been established that Jayne is a superb tracker, he could be the grizzly and super shaggy dog that finds Mal. I like to think that the Miranda experience, or seeing Book die, really serves as his Han Solo-like hero awakening and he's now all in.
Inara is off doing her thing. Is she even still alive? I don't think fans would take too kindly to yet another character dying. Kaylee and Simon could have a whole passel of critters underfoot. Is Zoe living relatively nearby to Mal? Maybe she's the only one that has any kind of clandestine contact with him - they have to tread very carefully because the Alliance is after them both. I don't know about River. It's really hard to wrap my head around her being not crazy anymore. Maybe she's stuck around with Simon and Kaylee. Sadly Wash and Book are (obviously) still dead. That brings me to....
What's Adam Baldwin's one worry about bringing back Firefly
I have read some of the comics - shamefully I've probably missed a few - and I don't recall Zoe being pregnant. But as I was reading this article that very thought occurred to me. Why couldn't you introduce a Wash Jr in the future? Why couldn't you have Alan Tudyk cameo in some flashbacks? I think I read somewhere about there being a competition between Wash and Mr Universe....something about Mr Universe getting the highest scores in flight school by cheating and Wash actually being the better, nay the best, pilot. I'd like to see more of that. I think a great gag would be to have Wash just starting to grow his mustache.
Joss Whedon explains how to (unrealistically) bring Firefly back
I'll admit I've scribbled more than a few pages of fan fic. And all the constant chatter of "what ifs" and "how would theys" really get the imagination going.