I've heard an interesting rumour that they're bringing the Master back at the end of this series. The interesting thing is that John Simm (from Life on Mars) is set to play him.
The two part episodes are called 'The Sound of Drums" and "The Last Time Lord". John Simm is credited as playing a man called Mr Saxon - but he is not who he appears.
Another rumour - just to confuse matters - is that this is just a cover, that John Simm will in fact be the Doctor's next regeneration. They're just using the Master story line to cover up why he's been in filming.
Either way, I'm good with these ideas. But I'd like to see the Master back, and John Simm may be a good choice.