Didn't we had a imperfect duplicate of a kryptonian in Batman v Superman? To me Brainiac is perfect as villain for the next Superman movie. They already have a plot with him in the 'Superman: Brainiac' storyline by Geoff Johns and Gary Frank. I'm pretty sure they are not done with Luthor and that we'll see him again, somehow. As much as I love Parasite (one of my favorite villains in comics), he is not a popular villain and I see him better as part of an evil society like the Secret Society of Supervillains, that otherwise would be a perfect menace for one Justice League movie.