I have to admit that I haven't seen MoS. What I heard about the story didn't really appeal to me and I couldn't have spared the time anyway. Maybe as a rental. So, that said...
Based on what we've seen in the previous Superman movies, suppose Gotham existed as it was in the Batman movies. Would Superman actually leave it that way? He's involved in rescues, forest fires, earthquakes, etc. throughout the whole world--why wouldn't he spend time cleaning up Gotham?
Needless to say, I don't like the idea of a crossover. Besides, I don't know if DC could do it "justice".
I agree with Christina's comment about Marvel letting each storyline breathe. But they could do it and still have crossovers. I don't know if DC has ever had a unified universe. Marvel has one universe so they didn't have to make story or character changes in order to bring them together. Even the Justice League stories seem to be independent of those of the individual heroes. For example, could DC marry the Batman and Green Lantern storylines? Marvel Comics could and I'm happy to see that they are making the movies the same way.
