I Am Not Responsible For This Brain

The actor who plays Lord Vetinari in the Going Postal adaptation bears a disconcerting resemblance to Roger Delgado.

I'd suggest a crossover in which Ankh-Morpork is part of a long-running gambit by the Master to a) take over the universe and b) blow raspberries at the Doctor, but I'm hard-pressed to think of a regeneration of the Master that could run a large city that successfully for that long (or a Master who would let Sam Vimes within twenty feet of him, come to that). Perhaps Vetinari is the Master's long-suffering, vastly more competent older brother? Occasionally they meet up and have drinks together at the Palace or the TARDIS or whichever planet the Master is trying to conquer at the moment, and Vetinari tries to impress upon him the finer points of Machiavellian scheming. And then Drumknott has to listen the rest of the day to Vetinari sighing that the poor young fool will never be a proper tyrant while he's still mooning over that odd ex-boyfriend of his.

...now I really want to write it.