how about Lupin or Pettigrew - they're the last members of the marauders - Pettigrew has a life-debt to Harry, and Lupin is probably interested in his wellbeing (maybe enough to travel with him in his quest, or die for him)
which would give JK a wider range of characters from which to choose to kill one and reprieve another two - and thats just what i think as being a main character
maybe Dudley will die to save Harry, then when Voldemort tries to kill him again the curse will backfire, and kill Voldemort (having had the horcruxes destroyed earlier in the book) - but this time even though the curse backfires, the residual energy of it kills Harry as well - it would make a nice twist.
since sirius didnt have the mirror with him when he went through the veil, i assume that it is in Grimauld place - therefore it is possible that at somestage when they are going through the stuff there (i think this likely because i reckon that RAB was regulus black, and the locket from book 5 that they couldnt open was the horcrux - and when they realise this and go looking for it in Grimauld place they find the mirror [but i reckon that Mundungus will have taken the locket] ) - then they can repair the one Harry has and use them to communicate