I would love to see QT write and direct a Western for Clint Eastwood to star in. Westerns would be a perfect fit for QT, and Clint would be the perfect guy to deliver QT's smartass killer lines.
I also think QT should make a guerilla low-budget film about Haiti or Iraq, sort of like Black Hawk Down but in a cinema verite style. There have been enough WWII movies -- why not do something about terrorism, Rwanda, Haita, the Middle East. Do something political, like Godard did with Le Petit Soldat and Les Carabiniers.