I'd always thought that 'Blindfolded Building' would be quite a good idea, you'd have people shouting the instructions: "Left a bit! Up a bit!" Sort of 'Changing Rooms' meets "It's a Knockout" (Via 'The Golden Shot' if you're old enough to remember that)
Maybe they'll do one about gardening for somebody else, or cooking for somebody else; there aren't nearly enough makeover, gardening or cooking shows on TV. To give it a new angle you could team up a member of the public with a celebrity and get another celebrity to judge it! Then the lowest...