Very nice idea.
I've only played part two and read about The Witcher 1, plus I did some reading about the books. Basically The Witcher 3 is about Space Elves and a young girl, now young woman, who is like Matilda.
I'm pretty sure the choices you make from W2 -> W3 are supposed to be a lot more significant this time. I remember people saying they spotted Letho in one of the trailers so... and I left the guy alive in my game.
Also someone from CDProject mentioned that somewhere in the beginning of W3 you'll be remembering the decisions you made in W2 through dialogue options, this was done so that you don't have to import your save files.