As far as I can tell, Watchmen fleshed out the backstories of its characters far too much to leave anything of value for these prequels. Rorschach was kind of dull before he lost his faith in humanity, Nite Owl's only interesting quality is that he's incredibly normal, Silk Spectre hated being a vigilante, and Dr. Manhattan doesn't really have a timeline after being blown up, at least not as far as he's concerned. Anything left out of the original Watchmen, unless I'm not thinking of something, is probably too dull to make anything out of, except for some unimaginative superhero comic where some cool people beat up a bad guy because they're just nice like that.