I can't really say that a human dealing with living forever is a common theme. However, Ashildr is designed as hardened by outliving everyone else, resigned to living for the adventure because she's bored, not caring about anyone else (or so she's convinced herself) - pretty much what we would expect because, well, that's what modern Doctor Who is, isn't it? That's what the main character does: gallivants around the universe and bemoans the loss of his people and his companions. He's been doing this for nine seasons now. So, this episode was basically telling the exact same story again, except in concentrated form.
Given this, the plot was pretty transparent and predictable: something she did would endanger a lot of innocent people and she would realize that she actually does care, and whatever that was was going to be solved by using the second chip. The only twist was that she was using the Doctor to get what she wanted.
I think what this episode was really missing was any sense of depth. Ashildr herself had no definable character or motivation: she vacillated between bored with her life, flippant to the lesser humans, begging the Doctor to save her, and... well, I'd like to say angry with the Doctor for having done this to her and left her, but despite the words she was saying, Maisie Williams certainly didn't back that up with any actual emotion or expression. She never actually conveyed any real despair about her situation. Perhaps all Ashildr was was bored and apathetic. If that's the case, then I guess Maisie did a great job.
Sam Swift was a Star Lord wannabe, not particularly competent as a brigand but full of sass and puns, and he was designed this way so that you'd sympathize with him. The only other "major" character was the lion dude, but he was just the token bad guy. Thus, in an episode which was supposed to be all about character and life choices, the two guest characters were simply bland. Add in the Doctor simply talking and talking and essentially only saying, "Ashildr, this isn't you," until other events finally slap her in the face, and it made for a very boring forty-five minutes.
It was also very disappointing, following the Doctor's "I made a terrible mistake; oh well, she'll just have to deal with it" ending from last episode, that he didn't revisit the mistake or express any regret/shame over it. The point of the episode seems to have been "the people you leave behind have to pick up the pieces of their lives on their own", ignoring the point of "hey, Doctor, you need to stop making impulsive decisions for other people and start paying for your mistakes". I guess I was looking for "The Waters of Mars" and instead got "School Reunion" (which made the same point much better and without so much blather).
Last observations: No one got even hurt during the alien attack. Not a single person. And chalk up another "show endangered/dying children for FEELZ" scene. :/