It'll be interesting to see who they get for the next Doctor, I second the thought that they'll go for a woman. I think Missy has been such a massive success for them that they'll have a lot more faith in the idea.
Towards the end, when Bill was in the TARDIS with the pilot/Heather and Capaldi was out on the floor, I thought for a few moments they were going to make the pilot the new doctor and reveal her as some form of Timelord we hadn't seen before.
It was just something in the way Bill questioned why she could fly the TARDIS and the response of something like, 'I'm the pilot, I can fly anything' that got me thinking...'Hang on a second, are they about to....?'
That could have been a neat way of doing it. Who says we have to follow the same Timelord the whole time.
Wasn't expecting the ending either. Really happy to see him back playing Doc 1 for real this time.