I don't think there is anyway that a Superman/Batman movie could work. Unless it featured them as allies and even then it would just be pointless to me.
I could see there being a pre justice league movie that features Superman and Batman as the main characters but there would definitely have to be other founding members involved. It would make no sense to me to have just Superman and Batman teaming up to stop some worldly threat without having other Justice League members making appearance. Having these two facing off against each other in a movie seems even more ridiculous to me considering the Superman character that was established in, Man of Steel. In order for a movie like that to work, one or the other would have to be portrayed as the protagonist, at least until the climax.