I like Maisie Williams, but she had way to big a part last season to now play the Doctor. It is one thing to cast someone who has appeared in a single episode, but they made her character a big deal in Season 9.

I personally want to see a darker Doctor.

A doctor that's more at odds with their identity and more vulnerable to the Master/Missy's attempts to join forces for the wrong reasons. Plus, having a weaker moral compass in order to properly examine the potential outcomes of different actions. You know? Instead of keeping the Doctor as some sort of bastion of righteousness, with the companion perhaps acting as an anchor for this more volatile incarnation.

I think it'd be a good way to get a look at how the Doctor sees themselves and how they came to be where they are now. Although I like Peter Capaldi's Doctor at the moment and hope he still has another few seasons left in him.