7. White Martians
The Avengers had the Chiuatri. The Justice League has the White Martians. As shown with the Justice League animated series, the White Martians are a great starter villain for the team, and a great excuse for them to come together. Now, it can be argued that the White Martians would be too similar to the Chiutari...but, unlike them, the White Martians can hide among humanity. There's the element of paranoia: anyone can be a White Martian. The other benefit would be that Martian Manhunter, one of the JL's most iconic members, would have to appear. You can't have the White Martians without him. Another interesting element would be Miss Martian, but maybe that could be saved for a sequel, after the White Martians are established as a threat. The point is that the White Martians are very interesting villains who can be worked into the story a whole number of ways.