might be interesting if it goes as the film did up to the 'releasing Callypso' part, then instead of dying, at that instant Eliz. Sw. gets all Chosen... also Callypso on departing might give her something 'extra' in return for what was taken. or stuff like that. Maybe immortality as a slayer?
Then that "one day every ten years" thing wouldn't be such a crock (based on the film, old Will has 2 or 3 good days, then she's dead...kind of a downer) instead what if they meet every ten years... for a long time?

I like this start. Hoping to see more of this story soon, because it has a great potential. :)