I still like the original idea, but this is an interesting tangent. It'd be difficult to fit into the framework of Series 4 as it stands at this point, though. Although - they did go to the supposed pleasure planet of Midnight, so theoretically, such a story could be set either before or after. And I can see him being terribly reluctant to return to Karn with Donna in tow and a still-existing bond to Rose. If anyone could get him to go back, though, it would be Donna.