I liked the story, but that does not mean that I liked everything in it. Now Bill may not be my favorite Companion (that's still Clara) but I do like her and having her become a Cyberman some would say a fate worse than death is not something I wanted.
Also the time dilation they got it...
Why would hundreds of people have to know we are not talking about the new Doctor being in all of the 2017 Xmas special just at the very end and they can film that part separately from the rest of the episode so only a very few would have to know. In fact, the new Doctor does not have to be in...
Because it should create a lot of publicity for the show and if people don't know who will be playing the new Doctor maybe, it'll increase the number of those watching if only to find out who he or she is.
But, the good news is if they don't start filming season 11 till early 2018 there is a chance to they could keep the identity of the new Doctor secret till the 2017 Christmas special.