I like Mr. Freeze, but I don't think he has the material to fit into a feature length film. His story always worked great in a half hour format. He's kind of a less dynamic character, as sympathetic as that character may be. And I don't think it's worth it to fit that type of story inbetween and around the greater arc of Two Face & Joker. Also, you'd have to give screentime to cryogenic freezing, sci-fi elements, and explain the suit. Seems like a lot of trouble for a simple story that may or may not mesh well with the rest of the film.