If DC began really pushing into the OGN market, and running it with a regular product supply, we'd stop seeing the books come out "whenever" and stop seeing such long wait times between volumes.
I assume that right now, DC waits to see the returns on the OGN's before approving the next volume. As a primary part of the publishing line they'd have to be more consistent than that, and keep stuff in the pipeline. They wouldn't really have any other choice, their expenses kind of demand a steady release schedule.
I'd think that two "full size/six issue" OGN's a year is perfectly viable. Same amount of content we get from the floppies. And I don't think maintaining interest would be that difficult a job either, people wait months between tv shows, they wait years between movies and novels. You'd just keep people's attention with early design work, previews, and interviews, same as novel authors. And it seems a growing number of people are already trade-waiting comics, and graphic novel sales are increasing. It'd be a different kind of medium than us Wednesday Warriors are used to, but I don't see any reason for it to not work. It's already working, and with a little adjustment and a sped up production timetable it could function as a larger part of DC's output.
I figure a publishing line of OGN's would include titles meant to carry a ongoing narrative (Batman season 1, season 2, etc) and between those they could drop some self-contained, stand alone stuff (Elseworld Batman, etc) to keep people engaged while they wait for the "core" story to reach the next installment.