But, so far as we know, the Rock itself is rather timeless (at least from our perspective). You don't need a huge origin story. You could still have a main plot with flashbacks and references to flesh things out. It's not like FR was all THAT dependent on their origins: and what origin would you talk about anyway? Proto-Fraggles? The ancient Fraggles when they had leaders? Or would you describe the Rock Fraggles separating from the Cave (actually, that might make a cool flashback)? Is the Weeba beast important? Why sum up the series' plot if you're going to have the F5 go out into Outer Space on an independent adventure anyway? Doc wasn't a big part of their lives until the end. He and Sprocket were just there. There's no reason we can't have the same concept with other Silly Creatures. If it is a sequel, you can reference the Gorg hostility without re-hashing it. After all, that'd be some grudge if anyone even cares about old hostilities 20-something years after the fact -- oh, wait, I live in the South ... never mind. j/k
My point is we didn't find out everything about FR in the pilot -- so there's no need to bring it up and spell it out in the movie.