Something I just randomly thought about. Obviously, there are some clear Second Doctor references this season with the Great Intelligence and Ice Warriors appearing. But I just realized something random about the three Claras.
There are two female companions exclusively associated with the Second Doctor. One was a super smart human from the future. The other was from the Victorian era. Does this sound familiar at all with regards to Clara? Now I know this doesn't give any hints of what's to come. In fact, it might be a reference instead of a clue, but with all the Second Doctor references, I wonder if that's more deliberate than accidental.