I'd love a cameo from Hugh Grant as the new muggle Prime Minister. It'd be a great nod to Love Actually, and Richard Curtis is certainly a friend to the HP universe. (I expected him to get a writer/director chair for HBP before they brought back Kloves and kept Yates on.)
How different would GOF have been if Curtis had helmed it? I guarantee Ludo Bagman wouldn't have been cut, and Rowan Atkinson would have likely played him.