Bill calling out for her mother while she's dying: I'd say right there we pass companion background establishing and move on to 'this is a thing.' The notable/sad detail about it to me is that this isn't like Clara or Oswin talking to her mother. Clara knew her mother. Bill doesn't. She has, by her own admission, made her up, from a few spare details and a box of pictures. And that's what she has to cling to when she's dying. That is . . . heavy stuff.

This is really sad as well because Bill doesn't call out for her foster mum or even the Doctor, people she actually knows. She calls out for a dream, an idea of a person she never knew. She calls out for the days that never were...

'The nice thing about life is, however bad it gets, there's always one last option available: dying well . . . This is the moment you've been waiting for since the day you were born. Don't screw it up now!' <--- I have this horrible feeling this logic is going to come back for us later.

It's Clara's lasting legacy (and Danny's too), I wouldn't be surprised.