Well I'm wondering that there may be someone who has died in the previous books whom Harry might get an opportunity to save in Mirror Mirror. Although he's not saving the actual person who died, in a way it will feel good for Harry to at least save them in another reality. Of all the people who have died up to this point, who do you think would be cool to see saved? I would start a poll but there are really a lot of people who have died so will just leave it up to you to write down, and why you'd like to see it.
I'm not really certain how to approach this question, if only because there are multiple ways to do so. There's the first, and most obvious, which has been discussed a lot below: "Who might Harry save in the Mirrorverse?" Assuming self-containment. The other way I see this question, and the potentially far more interesting(though possibly less-likely) is: "Who does Harry save in the Primeverse via retrieving their alternate self from the Mirrorverse?"
This reason I suggest this alternative spin on the question is the, honestly, if MM ends up being a self-contained side-trip done solely for the thrill of having Harry jump dimensions... it will be a waste. SOMETHING that happens in the Mirrorverse MUST effect what goes down in the Primeverse. Or, why does it happen at all? So! I put forward that Harry - in the tradition of Star Treck and Anne McCafrey's Dragonflight and lots and lots of other time-travel stories everywhere - brings someone BACK with him. Or, at least learns something MAJOR while in the Mirrorverse. Both? I don't know.
But, I figured I'd throw the possibility out there for people to speculate on. If it were to happen, I doubt it would be Morgan. He's not really the kind of guy to skirt the edges of the Laws.
NOTE: I couldn't find a pithy way to fit this last part in, so I'm just gonna put it out there, too. Another option could even be that Harry - in his fashion of doing silly things with magic - brings back someone who DIDN'T die yet. So, we get two of the same person, but with wildly different experiences over the last decade, running around.
Thoughts?