The trouble with a series as it gets older is it can feel like a tradition, and tradition is the enemy of suspense, and it's the enemy of comedy. It's the enemy of everything, really. So you have to shake it up.
Steven MoffatIt's a funny thing about stories. It doesn't feel like you make them up, more like you find them. You type and type and you know you haven't got it yet, because somewhere out there, there's that perfect thing -- the unexpected ending that was always going to happen. That place you've always been heading for, but never expected to go.
Steven Moffat