9. Earth Exile Mark 2
Yes, it's been done before, but wouldn't it be a nice nod to the past to see the 'Cox Doctor' on Earth working alongside UNIT as his Third incarnation once did? Think about it, and the more you do the more it might just make a degree of sense. He's comfortable in a lab environment and would be quite happy demonstrating to companion & audience alike as he went about his work- adding a neat ' live science' element to the show, with the satisfying pay-off that it would all lead to a nice easy resolution in time for the next week's episode/practical lesson deep in the bowels of UNIT HQ. Plus he's not the type who'd need someone to tell him how brilliant he is & pass the test tubes. He knows how to do all that anyway, and could probably carry out in-depth discussions with Kate Lethbridge-Stewart as he was mixing even the most complicated chemical solutions. After all most incarnations of the Time Lord have shown the ability to multi-task at least once, but this one could probably take it to extremes and still be just dandy, though not in Jon Pertwee's 'fashion' sense of the term.