Although topping Chris Nolans trilogy sounds insurmountable, if you listen to the fans (me! then it won't be impossible. Just improbable.
The most important thing is for Affleck to really give his movie it's own identity. Man of steel felt too much like it was directed by a bargain bin Nolan (sorry Zack Snyder!) and that it tried to hard to be dark and realistic. There was a reason the neck snap at the end of MOS was shocking, and not at all in the right way. However BvS was a huge improvement, as it had a very different style and tone than the Nolan movies. I do think that Batmans solo outing will need to be a little more realistic than BvS though, as it could be too similar too Tim Burtons films. Secondly, I think that we need a much more personal villain than Bane or The Joker, for some real emotional impact. Think Hush or Jason Todd in one of his many for
Batflecks movie needs sidestep all of the traditional superhero movie flaws as well, such as stuffing it full of one-dimensional supporting characters, and tonal issues.
Since Snyder isn't directing this time round, hopefully that means it should be remotely comprehensible. Unfortunately Afflecks movies tend to have a general lack of tension, which coupled with the huge sequel schedule could mean a complete lack of excitement. However Affleck does have an eye for visual flare in the mundane, so it's not all bad.
It may seem like Affleck has a mountain to climb to make his movie good. It may seem like he is facing the impossible. It may seem like he can't do it. But he probably can. Maybe.