The difficulty with Epic is, what can there possibly be that is So Bad it unites Daleks, Cybermen, Sontarans, et. al.? It can't just be a primus inter pares. We've had the Master back; we've had Davros back. Running through my mental Spotters' Guide, I wonder whether it could be the Black Guardian. He is the only classic Who villain I can think of that works. Certainly, he would fit the whole "darkness will fall" theme, and I like to think I can hear Valentine Dyall-esque tones in that voice to support my case. The Time Lords, collectively, or one such as Omega or Rassilon, are a possibility, but Moffat has preferred to turn away from RTD story arcs. If that hypothesis is correct (and of course, it probably isn't), then obviously we'd need a White Guardian to salvage things somehow. Which would be pretty handy, as everyone else seems to be a tad incapacitated just now.
I was pleased to see Rory back; the tragic ending between him and Amy would provide a more satisfactory resolution to their story, but somehow I doubt that's over yet, either. We still don't know who River Song killed, and presumably whoever it was, they aren't part of this particular story. Then again, I'm not sure where or when next week's episode will be set, if the Universe has never existed. But I do have the feeling that somehow, it will all be explained.