Currently, long-term solution for USE dependencies is to add relevant flags to package.use. However, with the current use of USE flags this is not really a good solution. Most importantly, once enabled, each flag is left in that state until it is disabled explicitly. It means that if flag was enabled just to satisfy a dependency, after removing the revdep-package user has to remember to disable it back explicitly. What I would really love to see is an ability to automatically enable USE flags in a 'oneshot' manner. In other words, I enable a specific switch and portage is allowed to switch USE flags to satisfy dependencies. However, the flags aren't stored in package.use but in some dedicated store (world-use?). When I run --depclean, emerge checks if those flags are still needed and removes them if they're not.
*** This bug has been marked as a duplicate of bug 258371 ***
