From both a publishing and creative standpoints?
As a publisher, I'd limit the books to one per character/team. Every book, with the exception of Batman and Superman (more on them later) would be 64 (story) pages, cost $3.50, and feature one one thirty page long story set in the main DCU, and one 30 page back up story, set during any time in the characters history. The remaining four pages would be dedicated to letter columns, fan art, text pages, etc. The Superman and Batman books would be 96 pages each, with one main story and two backup stories (each thirty pages), and cost $4.00.
The main stories in the books would be exactly like the current comic books, format wise. The stories will have a stable creative team handling stories of an undetermined lenght. The back up stories will be self-contained, last no more than three issues each, and be handled by a rotating creative team.
Each book that merits it will recieve two TPBs, one for the main story and one for the backup one. They will be distributed in both regular and digest sizes, with an emphasis on the digests.
For characters who cannot carry a book, there will also be an anthology book, called DCU, chronicling their adventures. The book would be 96 pages long, and follow the same format as the back up stories.
Creator owned montlies and miniseries will follow the current comic book format and if they merit it, be collected into TPBs once finished.
More on the creative side of the DCU later. So, what are your ideas?