Believe me, Matoi and Satsuki have fairly simple motivations (vengeance and conquest respectively) but they have complex personalities to back those motivations. To use an analogy in relation to personalities, one note on a piano can sound good but cleaver use of multiple can make classics. We can have a standard badass be Frankenstein but so many movies have that standard badass. We instead could have one who, while strong and durable, just wishes for a end to his life or a purpose since the original story was about a man reborn and looking for people but shunned because of the matter of creation. Also, the rule of movies is that we can't just have supplemental material provide stuff for the characters or plot before the movie (note, things made after the movie that expand them is fine but I don't think that having prerequisite reading does not speak well to the characterization or plot of a movie.) We would complain about this if it was any other movie so why an exception for Frankenstein
Also, why must we not have a bit of a snobbish behavior with movies. Media is starting to become a bit boring to me. I've seen so many tropes, cliches and bad executions that I just want to have something interesting but with good traits. Concepts are a dime a dozen, hell I can come up with a concept to have Shinto, Greek, and Hindu gods in war with each other giving abilities to "avatars" to solidify their power but that concept can be ruined if the characters are bland or one-note, the plot is a slog, or if fails basic editing.
I guess to put in simple terms, I want a movie made by someone with the imagination of a 10 year old but the intelligence and artistic mastery of a savant.