Interesting you should mention that. Right around the time you implemented multi-project pipeline visualization, we built a bot that listens for GitLab build completion events, hits the (undocumented /unsupported) internal GitLab global code search ElasticSearch index, finds downstream dependencies of the completed build (as declared in setup.py/package.json), and triggers their builds as well.. Taking full advantage of your nifty visualization.
So I think the feature we'd want to make that easier is an actual global code search API!
We do not have immediate plans to ship that yet, but I pinged our product team member there. However we do have plans to ship multi-project pipeline with an "inversion of control".
You will be able to specify pipeline relation with an upstream project, and when someone pushes a commit to it, a downstream pipeline is going to be trigger automatically. See an issue about cross-project dependencies - https://gitlab.com/gitlab-org/gitlab-ee/issues/1681!
So I think the feature we'd want to make that easier is an actual global code search API!