Justice League filming starts in 2016 so of course there has to be a big baddie for the team to fight. Down below are a list of potential suitors that DC's Justice League could potentially face off against.
Amazo is an android created by Professor Anthony Ivo. He has the capability of possessing any type of meta human abilities such as Flash speed, Superman heat vision or Wonder Woman lasso of truth. He big enough for the Justice League to handle because he a equal match for them and could take their powers and use it against them but he would be more of a "muscle" type villain since he has no real agenda and can only really be used as a weapon and not a "real villain"
If Doomsday isn't the "threat" in Dawn of Justice and doesn't appear in the film but is eluded to, then having him in the upcoming Justice League film works out perfectly. He a big enough threat that will take all the league efforts to stop and will push them to there limits to be able to defeat him. Now usually Doomsday is considered by most to be mainly a Superman villain but having him face off against the Justice League doesn't hurt as well and would be a visual spectacle on screen. My only problem with this is he will appear in Dawn of Justice and he will be the "threat" that will threaten mankind and will unite the Trinity in Batman v. Superman.
Darkseid is my favorite villain in the DC universe and is the perfect villain for the upcoming Justice League movie. He pretty much the most popular choice to be the villain of the Justice League movie and is a straight up bad ass. He can physically take on the League all by himself and could defeat them by himself, he Lord of a hellish planet named Apokolips and seize hold of a power named the Omega Force which turned him to a rock like creature. Darkseid is evil and intimidating and is a force to be reckon with but personally as a fan it too early for him to be the first villain for the Justice League, save him later on for the DCEU and hint towards his arrival on earth. He is rumored to appear sooner rather than later so keep a eye out on this bad ass villain.
Brainiac is the rumored villain to appear in Justice League Part 1 and he perfect first villain for the Justice League to face off against. Rumor is the space signal that Kal-El arrived in that attracted Zod will also attract Brainiac to earth. Brainiac is a galactic threat so him coming to Earth will spell big trouble. Now the non comic book reader will associate Brainiac with Ultron so it up to Snyder to try and make Brainiac as distinct as possible so people won't compare the two. Brainiac is a cold and calculating A.I that is a mental threat as well as sometimes a physical threat, he could easily hack in into any network on Earth and use it against the League and could cause some serious collateral damage, also having drones that the League might have to face off against. Brainiac the perfect first villain for the Justice League to go against, let hope he there for Part 1.