When egencache is generating metadata, is it already compares the previous cache entry to the new one. So, it has a perfect opportunity to generate a QA report for changed dependencies. The report might simply consist of raw dependency metadata which is intended for analysis by other tools. Classes of dependency changes that cause problems are enumerated here: https://wiki.gentoo.org/wiki/Project:Portage/Changed_Deps
This could also be implemented as a tool that creates and compares dumps of the metadata cache. The advantage of separating this from egencache is that the tool can run on a separate machine from egencache, operating on snapshots of the rsync tree. It might also be useful to blame specific commits, but that would be much more expensive because it involves generating metadata cache separately for each commit in which a dependency change may have occurred (could use git bisect).
The gentoo-repo-qa-bot commits are a convenient data source: https://github.com/gentoo-mirror/gentoo/commits?author=gentoo-repo-qa-bot
