It's hard to do a Firefly sequel without Nathan Fillion, but the franchise can survive without Mal as a Captain. Someone else ...

Kaylee, Firefly's mechanic, was at the center of one of the show's most controversial storylines, but she finally gets revenge for it in a grand way.

Not Joss Whedon's vision, but Firefly fans will probably like Alan Tudyk's idea of a revival of Nathan Fillion's series.