Recently I encountered far too many packages for which the latest stable ebuild was AGES old. For the vast majority of these cases the unstable ebuilds were perfectly functioning, if not better than the stable ones due to upstream bug fixes. Until now I used to report these cases as bugs, but I believe it's not very efficient.
Therefor, I would like to suggest that each ebuild would include a "Testing Period Expiration Date" -- a variable containing a date after which the ebuild would automatically considered stable, unless the maintainer chose otherwise. This will take a lot of work off the hands of package maintainers -- instead of having to look after every ebuild and mark it stable, they would only have to take care of malfunctioning ones, yet it would still allow a proper testing period before release.