I was re-reading the Secret Origins issue that had Golden Age Batman from about 30 years ago (was it really that far back?) , and I keep thinking I'd love to see more stories like that. A book (could be a limited series) set in the late 1930s giving us stories when Batman first went into action, before Robin entered the picture. A series that could re-tell / add to those stories from Detective Comics #27-37 and the Batman solo-story that got moved from Detective Comics #38 to Batman #1 instead (which really isn't that much material since those stories were about twelve pages or less per an issue), as well as introduce some new material between those adventures.
I know we also had the Matt Wagner-written BATMAN AND THE MONSTER MEN! series and BATMAN AND THE MAD MONK, but how long have those been out of print?