I would love a section with the typical archetypes of the era or character examples to streamline the process of character creation.
A supplement about gadgets and mad science is a must. I've never got the feeling that I could play one in the previous books.
Another supplement about how to create the batcave. Well, okay not just the batcave, you know what I mean. A book about how to create cool hero headquarters and evil lairs.
A book about the organizations and most importantly, how they interact with each other and the world at large. Give us a story and a system to make them...