We know that Doctor's character is changed with every regeneration, but we don't know where is the borderline.
He eats different (sometimes very strange) things. He is in different ages and looks. Sometimes he doesn't like soldiers, sometimes he is nearly one of them. Sometimes he gets in love or is touchy-feely, sometimes he doesn't show any emotions.
But, he always is good, he likes the Earth and people on it (although people may be questionable, when comes to some traditional series like the 1st Doctor). He is always very smart, always a man, always human-like.
On the other hand, his rival Master was evil in all of his incarnations, but
not always he was a man.
as we know.
So I'm asking, what can and what cannot be changed? Especially I am curious about the good-evil issue. Is it possible that The Doctor will regenerate once and he will no longer be good? Or are there some basic things that cannot be rewritten?