Another vote to at least try a pure historical. No hidden aliens. No sci-fi. Go on! Just have the Doctor go back in time and interact with history. It wouldn't fail!

Also not so much something to bring back but something I would like to see more of. An unknown threat. Some actual horror without a science fiction explanation to take away the scares. Yes the Doctors rather well travelled and knows alot. But how about something that even he truly fears and doesn't understand. Something that shakes his own beliefs about the universe. Something new and unknown and slightly supernatural. This seems to be anathema especially in New Who. But why? It gets kind of old when the doctor knows the reasoning and motivation of his enemies all the time. Scare the Doctor. And us!

I agree, but I don't think we'll ever see much of a global scope settings-wise. To credibly realize these places and times would require more exotic location filming and / or a lot of CGI effects. Not to mention large casts of non-caucasian actors, which is doable but not really convenient. It's all extra expense that they don't have to take really.

But it is something I would like to see too. Feudal Japan would be another great setting.