Also, Rowling will not be introducing any new characters in the 7th book.
And why do you assume that Regulus Black is the obvious answer? I think that maybe a few people remembered him from the previous book and then spread the word around to everyone who was not so. . .quick [not to 'burn' anyone]. So, yes, it mayb be obvious now, but I don't think it'd be that way if not for a few bright people and media sources (ie the internet)