I have to think her garden would have a lot more familiar faces floating around in it if she collects Who's dead.
Actually, the theme I'm seeing so far is that she collects people who have specifically committed suicide as a means of sacrifice, whether its for the Doctor or otherwise. For example, the robot in the first episode, he kills himself by leaping to his death. The soldier in the other episode sacrificed herself by distracting the Dalek antibodies.
I haven't seen the latest episode so my theory could be completely moot at this point, but so far that's what I'm seeing.
As for Missy, I think she might be a regeneration of River Song. I think that when the Time Lords gave the Doctor back the power to regenerate (perhaps even another set of 13? Or more?) that it might have had some kind of ripple effect and returned Rivers regenerations to her as well.
It's kind of a stretch, but I guess we'll see what happens. =)