The prequel for those not under the umbrella of the BBC.
Set pictures from the mid-season finale show returning monsters and potential allies. This is the day The Doctor finds out who River Song is. Who's going to war? What for? Secrets to this and much much more to come.
And as was seen in the trailer, the return of the Cybermen. These appear to be the Cybus Industries Cybermen and not the original Mondas Cybermen that were thought to return this season. With Steven Moffat this means nothing, but these are back at least for a bit.