5. Metallo
Thanks to his Kryptonite-infused metal body, and a prime directive to take Superman out if he ever rises up to take over Earth, Metallo would represent a genuine threat to the caped wonder, and his appearance would also fit into underlying issues hanging over from Man Of Steel. The military and political world powers clearly still distrust Supes, even though he proved himself by taking Zod out (but not before all of Metropolis was squished to a dusty pulp) and the introduction of a "super-soldier" programme to counter the inevitable threat would fit with the narrative of the first film, and Metallo's restrictions on Superman's powers could explain a hasty phone-call to Gotham.
Why He'd Suck Regardless of the presence of Batman and Superman, we'd still be looking at a dressed-up remake of The Incredible Hulk: the Abomination's storyline (with added Tim Roth ham) would be pretty much identical to a secret military programme to contain Superman, and as such, this sequel would suffer the same problems. In the interest of over-powering a villain without an enduring enough link to either Superman or Batman, we'd end up with a grotesquely enhanced vulgarity with very little emotional impact. Metallo would effectively just be a glowing green terminator, who couldn't command the same sort of resonance as a truly memorable villain, and the suspicions and interference of the military against Superman would feel like a step back to territory that was more than adequately covered in Man Of Steel. So, while he's a reasonably arresting character, he just wouldn't work here.