I'm still hooked on the thread dangled by the end of series three: As the Master's pyre burns, his ring falls from the corpse and is picked by by a female hand wearing red nail polish. As she lifts it, we hear the Master's manicial laughter.

Master not dead yet?

And where was the Tardis that took him to the end of the universe?

Hmmm.

Maybe the Doctor is the last of the Time Lord's just yet?...