If RTD hadn't fenced in the current Whoniverse to be devoid of Time Lords that could be sniffed out a mile away, I would like to see an arc where The Doctor is forced to chase down a regenerated Meddling Monk and Rani working together to conduct the perfect experiment to screw up historical events and timelines across multiple worlds and time periods. They would need no real purpose for doing it other than to be "anti Doctors" and run loose. If you are familiar with author Harry Turtledove, I would like to see a backdrop of alternate 'what if' changes to the existing timelines and world events that have to be strategically undone by the Doctor through tough choices that fix time. Imagine- Tardis chase scenes, historical puzzles, timey wimey messes created across the cosmos. I would like to see James Nesbitt as a slightly insane but crafty Monk, with the Rani played by Keely Hawes from Ashes to Ashes, who likes the Doctor but also wants to show him she's smarter. Each Ep could stand alone and serve the overall arc, you would see how complex the puzzle really becomes. Borrowing from Batman, sort of the Monk as a Riddler type, and Rani as a Catwoman type figure.