How about: A combination Outer Planes setting book/Great Modron March adventure? The book could follow the March around the Outer Planes, giving a short (say five to seven page) adventure set in each plane due to the March (advancing in level as the March proceeds around), while dedicating another five to seven pages to an overview of each plane (with interesting locales). Add in ten pages or so for Sigil, and say 20 pages for monster updates, and that would pretty much fill up a full 256-page book.
It wouldn't quite be a full adventure, and it wouldn't quite be a full setting source book, but it would be a good combination of both and would appeal to a wider audience.