When I saw the gas-filled cage from last night's Doctor Who, I really thought the villains from that other Peter Capaldi Doctor Who project were going to be back. Not the Pompeii one. The other other one. (Children of Earth spoilers in the video.)
In retrospect, I'm kind of glad they didn't bring the 456 back. I think Children of Earth is legitimately some of the best Sci-Fi television of the past decade, so I'm glad they kept the mystique, but the holding-cell-thing (especially in the preview) really looked like the 456 room.