I think I may have just figured something out which could make book 7 extremely interesting.
SPOILER ALERT
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Okay, so for those of you who have read the book, you know that Dumbledore and Harry find a locket in the cave. The locket contains a note stating that it is not the Horcrux but a decoy, and that the Dark Lord can look forward to losing another part of his soul as it is going to be destroyed by one "R. A. B." The question is, who is R. A. B.? I believe that it is Sirius's younger brother who somehow managed to escape Lord Voldemort's judgement. Regulus A... Black will become a new addition to the character list in book 7 and will eventually meet Harry and most likely aid him in defeating Lord Voldemort.