The DCEU is being formed leading up to Justice League and DC are known for great Villains but which ones would the Justice League face because they are pretty much gods for they need god-like villains.
As DC villains go, they don't get much more universe-threatening than the Anti-Monitor. The Anti-Monitor hails from the anti-matter universe and is obsessed with destroying all other universes and it nearly succeeded. It was the villain and cause of the Crisis on infinite earths comic series. Even now, it remains one of the most overwhelming enemies of the Green Lantern Corps.
The Anti-monitor is a possibility for a Justice League Movie because how over powered it is. it needs every superhero and that's not a understatement. In the comic series. Crisis on Infinite Earths, it needed every hero from every universe. Now I'm not saying they have to do a multi-verse story with the villain but this is the villain that could connect DC's TV and Movie universes
It is surprising that Brainiac hasn't appeared in a Superman/Justice League movie. Aside from Lex Luther, Brainiac is without a doubt Superman's greatest foe. The cold, unfeeling android has devoted himself to collecting every last bit of knowledge there is. He catalogues the many cultures of the universe, collects samples, and destroys the rest so no one else can possess what he does.
Darkseid is one of the most powerful beings in the DC universe. He is the evil ruler of the planet Apokolips, and his ultimate goal is to conquer the universe by eliminating all free will from the universe and reshape it into his own image.
Darkseid is DC's Thanos except he doesnt need a Gaunlet to conquer the universe. He is likely to be the Villain for Justice League part 1 and if you need convincing just look up Justice League: War.
Hro Talak - The Thanagarians
The Thanagarians are the alien race where Hawkman and Hawkwoman/girl are from. Hro Talak is a character made up for the animated show and was a leader of the Thanagarians army. As part of their history, they are a warmongering race. In the animated show, The Thanagarians came to Earth posing as mankind's only hope against an alien race called the Gordanians who were on their way to Earth in order to enslave it. Under this guise, the Thanagarians planned to use a device that would both destroy the Earth and give the Thanagarians access to the Gordanians home world, allowing them to once and for all destroy the Gordanians.
Using this as a starting point, they could make a very interesting and different story of ideals and goals. Killing one planet to allow them to stop a wa could be a way to introduce Hawkman and Hawkwoman as traitors to their own race.
Doomsday is portrayed as a deadly monster born from the depths of ancient Krypton. His creation imbued him with few feelings, mostly hate and destruction, which led to his destroying worlds and eventually finding Earth where he meets Superman. The character is best known as Superman's murderer in the 1992 storyline, The Death of Superman.
This character is rumoured for Batman V Superman, but should be in Justice League instead, he should be a Justice League villain Doomsdays the character who killed Superman. The world would need a collection of super strong heroes to stop him and could be the reason a JL is formed.