Becoming Insane said:
Any one of the Discworld books. (again, would have to be done right)
Already done. Hogfather, Colour of Magic and Light Fantastic have been adapted to TV films/two part mini-series to great effect. Colour of Magic and Light Fantastic were amalgamated into one feature, starring Tim Curry, Sean Astin and David Jason. Oh, and me. Okay, I was an extra, but whatever. Reportedly, the next in line is Going Postal. Sam Raimi has the rights to make a film based on the Wee Free Men, but apparently because of scheduling problems hasn't done anything with it yet.
Also, at least three computer games have been made of Discworld, all point and click adventure games.
I think the Eisenhorn series by Dan Abnett would be excellent movies. All very cinematic. Lots of action and visual spectacle that would translate well onto the big screen.