6. Amazo
I know, if you're a comics fan, it's not Ivo who brings the Manhunter to earth, it's Professor Erdel. But, as creative licenses are taken, if it becomes Ivo's desire to bring forth an alien life form in order to replicate it's abilities to be translated into his own creation for global domination, then that ties Manhunter's arrival nicely into a possible villain; Amazo! When it comes to comic books I can be a bit of a purist. Sometimes I want things to be exact, but then I remember to get off my high horse and let the appropriate path be paved that makes logical sense for the film. Having Starro as the first villain would be fan service, but suicide. What a difficult character to try and make sense of and deal with. So why not just jump to the villain capable of mimicking and using the League's powers against them? That'd be the android Amazo, created by Professor Ivo. Perhaps Ivo shares the sentiments of the people of earth, that these God's are simply too powerful to walk amongst men and they must be destroyed in order to keep balance in the Universe. In order to do this something must be capable of destroying them, so he creates Amazo out of what all mad scientists believe they have, "good intentions". But Amazo is too much for anyone to handle, even the League at first, and proves a mistake. He's the entire Justice League wrapped up in one. That's dangerous and that makes for some incredible battles. Sub-fights can take place with Ivo's other androids, etc. but it's Amazo that becomes their greatest foe, who decides rather than restoring order to the Universe he'll just destroy the League and assume his right as it's ruler. Only makes sense for the free thinking android of a mad scientist. The types of battles we could see with the League all working together to stop a single villain is pretty incredible. It's them fighting their own abilities, which causes them to admit to one another of their own personal weaknesses in order to, essentially, defeat themselves. There's some beautiful character development in there, considering the League would need to come to an understanding that to defeat their greatest threat they'll need to surrender themselves to one another, exposing their weaknesses and fully letting down their guards in order to work together as a perfect team. Everyone would be hesitant, but trust goes a long way, and the sentiment of the team being unified as one based on their trust of one another is incredibly important and also distinctly different from the Avengers.