wow, it has been a while since i have read any good books worth making into movies.
IMOP, the HP series didn't need to have screenplays written (or even the movies made, by extension). One book that i would like to see as a movie is Ender's Game by Orson Scott Card. damn good book, but due to the huge amount of antigravity/outer space scenes in the book, a lot of CGI would need to be used.
also, the Weather Watcher series might make good movies. i haven't read any of them after the first book, but it was good and kept my interest the whole 5 hours i was reading it.