800 million is impressive. But seriously, if a film with Superman AND Batman only makes like 150 million more than a solo Superman film then it isn't showing much potential for growth. I imagine they'll continue with Justice League films and Superman and Batman films... but i think they'll start second guessing expanding beyond that with other solo heroes.
I want to see solo Wonder Woman and Flash films... but if BvS isn't a mega hit i think they'll get cold feet again and just stick to Supes and Bats, using the other characters in JL.
800 million would be low end for a movie starring the biggest superheroes Warner Bros has. So on that point I agree with you. Will BvS gross a billion? That's hard to do. And so many things will play a role in the outcome. Even trying to guess what it will do when 2016 is so far away (March, no less) is difficult. But WB will need all the momentum it gets especially with Justice League soon to follow. But ultimately superhero films sail upon how the actors and story plays out with the audience and too often the end result doesnt match the resulting hype and speculation prior to the movie being released.