Obesity: I know Morgan covered the topic very thoroughly in "Super Size Me", and the thin-person-in-a-fat-suit bit has been done to death - but still, it would be interesting to see someone who has never been obese try to live like it for a month.
Jews and Palestinians: I'd actually like to see this as a double-shot - have a Jew and a Palestinian live with each other's family at the same time. It would be very topical!
Homelessness: May be too close to Minimum Wage in terms of subject matter, and may be just too difficult to do. But it'd be interesting.
Disabilities: Maybe find someone who thinks the handicapped get all the breaks because they get good parking spaces and can skip amusement park lines, and have them in a wheelchair for 30 days.