I think Watership Down would make a wonderful musical - and family friendly to boot. I thought I heard long ago that someone was going to do that, but I never heard that anything came of it. Does anybody know?
In the current popularily of Jane Austin, I think Sense and Sensibility or Pride and Prejudice would be interesting.
For movies, I think Big Fish would be interesting, and I completely agree with Finding Neverland.
"And the postman sighed as he scratched his head, you really rather thought she ought to be dead..."