4. The Whispermen / The Silence
http://www.youtube.com/watch?v=vvZMSrktskw They're two aliens similar in design and both in Moffat episodes featuring the conundrum, "Silence will fall when the question is asked". But what connects both the Silence and the Whispermen is actually ingenious. The Silence are trying to prevent a question: hence why they're called "The Silence", and hence why they have recognisable facial features, but no mouths. The Whispermen are the complete opposite they're trying to force the answer, "Doctor Who?" from the Doctor. They have no eyes, but mouths. I know, you're probably thinking "That's not a clue." Well, we saw preview shots in trailers and promo material of both these monsters, and both the Silence and the Question were teased a long time ago. We could guess that they'd be related, couldn't we? For example, the Doctor's been deleting himself from databases, and we've still not got an answer about that: if you see an alien in the Series 8 trailer who has some link to information - maybe the GI's return? - then that's another clue to the arc continuing.