As we now Batman vs Superman is DC's starting point to creating a DC cinematic universe and is rumoured to introduce all the Justice league characters in some way to make people interested in the characters.
At this point it is believed that there are going to be 7 Justice League Members being Batman, Superman, [Wonder Woman](movie:45787), Flash, Green lantern, [Aquaman](movie:264237) and Cyborg, but I believe there is a character that will appear but not technically be a member until in Justice League Part 2 in 2019 and that character is Hawkman.
The reason I believe that Hawkman could appear in this universe not just cause the character would fit in with the theme and style that they are going for and that in the Man of Steel prequel comic they mention the Thanagarians which means that they are apart of the universe already but cause it would better help the story, for example Batman fighting Superman now people have been saying "Batman can only beat Superman if he had Kryptonite" which is not true if he had Nth metal which is the metal of Hawkmans race (Thanagarian) which is an near indestructible super metal that is weightless and increases strength and can harm superman which means that if batman made a suit of armour he could possibly take on superman in a fight.
Another character that uses this metal is Deathstroke who is rumoured to be appearing in [Suicide Squad](movie:2283363) which would amplify the fact of him being a threat and why/how he can kill as many people that he can without being injured or killed himself.
and if they did use him that means that the Thanagarians could be the main threat of the [Justice League Part One](movie:401267) & Two (or a Justice League sequal) and Hawkman joins them because he believe what the Thanagarians are doing is wrong and fights against his own kind and becomes the 8th member.