The upcoming Roseanne and Will and Grace revivals will both pretend the series finales never happened, and the Wet Hot American Summer on Netflix just went ahead and used the original cast (now mostly middle-aged) for a prequel series.
It’s not out of the realm of possibility if the stars somehow align, as there is precedent for projects using do-overs and timeline shenanigans for revivals.
Of course, even if there is someday a Firefly revival, we won’t get Mal and Wash back together without either ignoring the events of the Serenity movie or setting it chronologically before Serenity went down.
Fans are often willing to fudge the details if it means recapturing magic, and I’d say that Firefly devotees would be on board for just about anything that brought the cast back together.