I must have missed your complaint that Capaldi's hiring implied that the only way to make a good Scottish character is by changing a young English man into an old Scottish oneI really haven't seen anyone at BBC imply that and don't see how the casting by itself would imply that
I can see where he is coming from on some aspect of this. Not all but some. Now the Time Lords I disagree with as they are not Human thus our gender roles don't apply to them. But I have seen this in other shows like in LGBT issues where a character suddenly becomes Homosexual or Bisexual or some other sexuality that they were far from originally depicted.
So yeah, when changes feel too forced it feels like pandering. I don't mind if its done early in a series as creators and writers find their feet with a character but random later changes can feel forced. I don't mind this change because it was established when the doctor first regenerated that he was a completely new type of doctor and each doctor since has a different personality to their predecessor on a Human level but still being the Doctor in a fundamental level. We see this in how the behave and how they approach problems and how they dress even so it doesn't feel forced to me.
Somehow I thought that the new doctor was going to be a lady when they brought in Capaldi, but I don't recall why I thought that.
I think that the whole regeneration with Mat Smith thing wondering if he was female and/or ginger started widening the door for a female portrayal so was natural for people to wonder and discuss the possibility. So you were probably seeing the rumour mills.