I like the idea of a spin-off based on the life of Voldemort. But I think to get the true feel of it, it has to start off in the first film of the trilogy all the way at the beginning, when Tom Riddle hasn't even been born. It should start with the Gaunts.
For us to get the real feel for Voldemort's hate of muggles and mudbloods, we have to see where he truly came from. You can't just start with an odd little boy in an orphanage. Sure, that would explain a bit of his evil nature. An orphan boy, loved by no one, thought of by all of the adults around him as deranged, strange, and twisted.
But to truly understand Voldemort, we should see the life lived by his mother, Merope. Tortured by her father Marvolo, and by her brother Morfin. The first movie of the trilogy, or even longer series if necessary, should end with Tom Riddles birth and the death of Merope.
I think this would make for a great movie. The following films, rather than following the year 1, year 2 them of the originals, would need to encompass rather than what year Voldemort was in, the levels of transformation. Perhaps the 2nd film would involve him growing up in the orphanage, done quickly in the beginning of the film, visited by Dumbledore, and then perhaps end with his little discussion of Horcruxes with Professor Slughorn, or somewhere about there.
The 3rd film in the series then, would encompass Voldemort finding all of his beloved items he feels are necessary for him to become invincible, and creating all of his horcruxes, culminating in his murder of the Potters and Harry's survival. Maybe even ending with the beginning scene of The Sorcerer's Stone, a fitting end and lead-in to the original series.
What do you think?