How about this --

We can predict anything, not necessarily a death.
Each statement counts for one point. If correct, it's a +1; if incorrect, it's a -1.
Each statement must be 100% correct or it is false.

For example:

Voldemort will die or will receive a dementor's kiss (same thing).
Harry will vanquish Voldemort.
Harry will not make the final blow that kills Voldemort.
Harry will survive.
Peter will die.
Peter will help Harry.
Something big will happen on Halloween that will allow Harry to communicate with dead people (perhaps a near-death experience).
Snape will die.
Snape will be a hero.
Dumbledore did not "die," in the usual sense of the word, at the end of the sixth book.
Dumbledore will not "die," in the usual sense of the word, during the seventh book. It's complicated.

Abs -- It's your game. What are the rules?

One more thought --
Epilogues don't count. If I say Harry survives, and the end of the book mentions him dying at the age of 136, that doesn't count. Unless specified by the person making the prediction, we will assume that all predictions are about the main story and the war with Voldemort.
-- Sound good?