Unrated,
I cannot possibly see Voldemort as Harry's Grandfather. First off Tom Marvolo Riddle did not have kids. He is also not old enough to be Potter's Grandfather. Sirius Black ran away from home at 16 and stayed at the Potters' from time-to-time. To quote Sirius, "I was always welcome at the Potter's". He was the same age at james Potter, they were in school together.
The time line makes the Grandfather link totally impossible.
I still say Dumbledore transferred his 'greatest wizard in the world' powers for the purpose of good, to either Harry, Neville or Snape - as Professor Snape may well be actually good.
Voldemort and Lucius bite it, as does Ron or Hermione but not both.
Potter will also be too young to be headmater, whereas Snape will be perfect if Dumbldores powers are transferred to him.
On another point, I can't see Ginny copping it, because, for the story to end on a good note, Harry would likely have to have a love interest. Not absolutely nesessary, but pretty much. Unless the book is 2,000 pages, I can't see him finding another and marrying etc.