An idea I read on the HPMoR subreddit that I don't remember finding here is that "the very stars in heaven" could refer to the Blacks (Every last one of them that we know of has star, constellation or galaxy-related names, including Draco). Hermione also offered "the skeleton is a key" as a hypothetical for what a prophecy that means "Susan Bones has to be there" might sound like, and Hermione did study prophecy on Harry's urging, and we know that Hermione retains book knowledge much better than Harry, though this is still rather weak evidence for a stars -> Blacks style riddle. It did seem pretty unlikely that Belatrix/Sirius would have a reasonable way to reenter the story in the time that remains, but that particular interpretation of the prophecy does point that way--and they are unclosed plot parentheses in the story's final stretch.

Side note: Narcissa was a Black by birth (Belatrix's sister, in fact), and "stars in heaven" is, as other readers have pointed out, an odd phrasing for what would normally be called "the heavens", but not particularly odd if heaven = happy afterlife or wireheading.