Of their current IP, there are very few that would really fit with an M rating.
Metroid could work but at the same time what would the M rating achieve that couldnt be done with the T rating of the Prime games?
According to the ESRB:
Teen-Content is generally suitable for ages 13 and up. May contain violence, suggestive themes, crude humor, minimal blood, simulated gambling and/or infrequent use of strong language.
Mature-Content is generally suitable for ages 17 and up. May contain intense violence, blood and gore, sexual content and/or strong language.
So what would really change about Metroid when going from T to M? Its a game about isolation so sex & swearing wouldnt make much sense. Essentially extra gore would be about it and thats largely unnecessary, gore for the sake of gore wont really add to the experience.
When the herd loses its way, the shepard must kill the bull that leads them astray.
