In some cases like bug 622462, emerge suggests mask changes when only USE changes are necessary. I would be nice to have a --autounmask-use-only option so that only USE changes are allowed (--autounmask-keep-masks allows keyword changes that are not necessarily desirable).
Patch and pull-request in progress. Just need to finish man page adjustment and write a quick unit test.
Since the first implementation of --autounmask-use-only also allows license changes, it's a misnomer. Instead, I propose a --autounmask-keep-keywords option. Patch set for review:
This is in the master branch:
Would it make sense to document an order in which portage should try to change configuration like USE flags (profile, make.conf, command line) and build command line options around that. It seems to me that changes like this are adding more and more options to emerge which kind of obfuscate it.