We all know that Batman vs Superman: Dawn of Justice will pit DC's two greatest titans against each other. But even a battle as epic as this one surely will be can't possibly consume the film's entire run time. Especially not if Lex Luthor has anything to say about it. With confirmed appearances of middleweights like KGBeast and Mercy Graves, as well as the rumored and presumed appearance of heavyweight Doomsday, is there anyone else that Lex might want to add to his band of maniacal minions?
There's a major player's name missing from the above villain roster: Metallo. Yes, the kryptonite-powered Terminator that is so susceptible to being defeated by what Lex Luthor once angrily termed "a clown in a Batsuit". Now, here's why I think Metallo needs to be in this already uber-crowded film. Actually, bringing him in at this exact juncture makes a boatload of sense for several important reasons. First of all, he's a great character. Second, his appearance could force Batman to save Superman. Thirdly and finally, unless in a fit of desperation, Lex brings out his purple and green power armor, Metallo's pretty much the only villain besides Doomsday that could really make things difficult for the Man of Steel. Now, if Metallo DOES appear in the film, how can that happen? Lex can't just pull a kryptonite-powered android out of a hat and say, "I built this guy because the fans demanded it." Well, I guess he could, but it would go against the whole tone of the film.
Now just imagine for a moment that as a direct result of the events of Man of Steel, there's been a rapid rise in vigilante activity, including the most well-known vigilante of the time, Batman. And naturally, the government will recognize that they're going to need a new defense system in place in the event of future attacks from outer space, (seeing as they kind of hate the guy that saved their ungrateful hides the first time and all). And who better to build such a defense system than America's new golden boy, Lex Luthor? Suppose that Lex is contacted by the Department of Defense and asked to begin construction of a robotic threat response unit to deal with extraterrestrials when needed and in the meantime clear the streets of vigilantes. Lex would then construct Metallo as the first prototype, but secretly install in his brain a hidden directive to eliminate the Man of Steel at all costs.
And so, suddenly, the prototype "mysteriously vanishes" and Superman finds himself at the android's mercy. But then when things look their darkest, Wonder Woman and Batman both arrive on the scene. Seeing these new players enter the game, Lex will likely dispatch Mercy Graves and KGBeast to deal with them. But after they are quickly subdued, Lex realizes that if he wants to win, he will have to break out the heavy artillery. The three heroes defeat Metallo and then are confronted by Doomsday. Batman and Wonder Woman attack immediately, but Superman is still trying to recover from his bout with Metallo.
Finally regaining his strength, he saves Batman from being killed by Doomsday, causing the Dark Knight to grudgingly admit that he was wrong about Superman. The two shake hands as Wonder Woman looks on in obvious sarcastic disdain. Suddenly, as a final act of desperation, Lex himself appears in his green and purple power armor (having reverse-engineered it from Zod's armor), and is summarily defeated by the three new Super Friends. At this point, the group all head out in full costume to celebrate their victory in the best style possible: over a latte.
As the trio is walking out of the coffee shop, however, Amanda Waller and her government goonsquad show up and send the Man of Steel packing, causing him to go into exile in his Fortress of Solitude. Batman and Wonder Woman are saddened by the loss of what they believe to be a valuable ally, but Amanda Waller then informs them that the UN wishes to create a superhero team called the Justice League and wants them as its founding members. This, of course, will pave the way for the Justice League feature film where the fledgling super squad are threatened by Darkseid. Realizing that they will need extra backup, Batman will send for Superman, who will arrive to help, and after aiding in Darkseid's defeat, be given membership in Justice League, much to the chagrin of many politicians, including Amanda Waller.