It would have to be set in the 1950's, given Harrison Ford's current age. And this would be a great opportunity for Lucas and Spielberg to do the perfect Fifties-era adventure story, with an older and more "beat-up" Indiana Jones. They've already done the Thirties and Forties, and paid homage to the great serials -- so the Fifties era of low-budget adventure stories seems like it should be just right for the "Indiana Jones" treatment.
Legend, oh legend, the third wheel legend...always in the way.