The usual relationship assumed for space dust to "living molecules" is the notion that fairly large and complex molecules may be formed in space dust more easily than here and that those large molecules, once they filtered down, may have been the "precursor" molecules from which DNA originated.
Since the evolution of new molecular forms is NOT something that happened a long time ago and then stopped, but is something that continues, "space dust" molecules arriving after DNA became common could be incorporated into existing molecules. Since this has presumedly been going on for quite a long time, the likelihood that the capsule will bring back something that hasn't already been here many times before is probably "extremely remote."
There's little reason to expect to find a new "godzilla molecule." The comet dust may be expected to contain stuff already here - billions of times before. With the new dust in hand from there, we'll be able to tell which of the crud we live with came from "out there" and to separate it from the homebrew of stuff it's contaminated with in earthly samples. Without contamination by common earthish stuff, we should also be able to learn more about the "pure forms" in space, to separate the specific components in space from earthly stuff that includes those same components from space.
Still, when quoting odds, one might well recall those famous (last) words of the great odds-maker, General Sedwick: "Those bastards couldn't hit an elephant from th_."