Pick a subset of Debian that is the core. Release the core only when it's ready, which should be more frequently than when all of Debian is ready. For the other stuff, users would have to [FIXME: do what?]
- Shorter release cycles - don't have to wait for Gnome/KDE/insert-big-and-not-essential-package-of-choice-here.
- Package pool could theoretically be reduced to a few megabytes.
- Users are on their own with most of the applications, since Debian's archive is de-facto reduced to just another pool with almost no QA (3rd party pools can do the same).
- Security updates only for core packages, or the need for another security infrastructure (when and where to release security updates for non-core packages?).
See ReleaseProposals for alternatives.