I agree with nvdoyle on Serenity. For me, IDNH. If they made a new series, I'd suggest just ignoring the movie events as apocryphal, or doing a "before the film" series with the old cast. But alas, with the death of Ron Glass, that in between series is out of the picture.
Doing 20 years later, you could get away with Mal and crew, minus two, and an adult addition…the grown child of Zoe and Wash (or fudge the dates, and a teen/preteen child and call it less than 20 years after).
You can drop the Reavers (never really liked that element anyway), bring back the Blue Hands Group, and throw in a surprising element from Book's past (maybe he *was* a grandpa, after all).
Not sure what to do with River and Simon, Kaylee, Jayne… I'd love to play with Mal's statement to Inara "Can you imagine? Me with a passel of youngun's underfoot?"
But kids all over the place risks the Crusher curse.
Maybe the less change, the better.
Books (and graphic novels) at least allow for the immediate aftermath of the film, or even the before-the-film, never-made tv season stories fans are really longing for.