How the Doctor falls... should have ended.

I disliked the plot with the Masters but....

I have never been keen on Missy

Have puddle girl, appear, after the final battle and offer to save Bill.
 Bill,  refuses to leave and begs Puddle girl to bring the children to safety first . Thus Puddle girl rescues the kids. Bill dies in the arms, of her beloved having sacrificed herself for the greater good