1: He's Brainiac: I think every fan will agree with me when I say Brainiac, next to Lex Luthor, is the archnemesis of Superman. He's been around for almost as long as Supes has, and has made numerous impacts on both Superman and Clark Kent's life, including causing the death of Clark Kent's adoptive father, Jon Kent!
2. We should have had him already: Are you telling me that in 5 Superman movies we still haven't seen Brainiac? And no, that one computer thing doesn't count. I mean, we had General Zod for God's sake! Now, don't get me wrong, I love Zod, but he shouldn't have been the villain till at least the fourth movie. It's about time we should get Brainiac.
3: He's an actual threat: Anyone who read Superman: Brainiac knows that Brainiac is more than a match for the Man of Steel. In addition to the psychological threat posed by Brainiac, we'd also get to see something we've been strangely denied: an actually good Superman fight. And I think the people will agree with me when I say that seeing Superman actually fight a villain in his movie would be a breath of fresh air.
I've gotta go, so I'm only gonna post this part of my list. I'll upload the second part, but until then tell me whatcha think in the only place you can.
Ciao!