This could be a spoiler, unless you've read it somewhere else:
The "Master" actor John Simm has been seen on set for the final David Tennant story. Simm's character, of course, was last seen dying in the season 3 story "Last Of The Time Lords," before dying and being burned in a funeral pyre, only to see his ring rescued by a mysterious female hand.
Is Simms back as the Master? Or is it a regeneration flashback? Only time will tell.