I'm pretty big on sticking to comic book origins, but there are some that need to be slightly altered or changed completely. One would have to be Bizarro. We can't have him walking around saying "me save city by destroying it." That just doesn't work. It would be horrible and campy. He could be crust-faced and a flawed clone, but he needs to not be a moron. Not only would it be a bad story, but it would also be horribly politically incorrect. Essentially it's a mentally handicapped person tearing stuff up. That just won't work with fans.
Metallo has a lot of potential, as does Parasite. Metallo is a strong enough character to merit his own story (it could be tied to Lex, but he's capable enough of making his own plot). Parasite would have to be done perfectly in his own story, but he would work well as part of a duo.