Problem 1: by who's standard do you rank them? Who decides if Spidey would be and all rounder or a speed freak? He's not as fast as the Flash but he is a lot faster than Batman. As such, each Marvel and DC character would have to be modified to conform to this. In fighting games you usually have roughly three 'classes', your fast characters, your powerhouse characters and your all rounders. For one thing as Hulk Power suggested, they are two different 'universes' so to speak so you have differences in power and scale. I've heard that both companies have discussed it with each other but it just wouldn't work.