I hope it isn't the usual anniversary thing of a multi-Doctor story.
The only one of those that really worked was The Two Doctors (which wasn't even an anniversary show) even if you had to invent a whole unseen branch of continuity to explain it.
A multi-Master story would be much more fun, if only to see Sir Derek Jacobi do his stuff again.
Could they afford Eric Roberts?
I'd imagine it would be hard for Simm to camp it up with Eric in the frame.