I was looking up movie scenes with mirrors over the weekend for composition inspiration and found these:
I also found this article: “The Greatest Mirror Moments in Movie History” from the Telegraph.
These are screenshot from some of the movies mentioned in the article:
The background from Taxi Driver (1976) might be useful for inspiration also. The centered composition works well for the horror tension, like in Occulus (2014), but I also like the composition where the character is framed to the side and reflected in the mirror.