Thanks for this proposal! It might be a cool feature indeed. I created an issue about it in our issue tracker, see https://gitlab.com/gitlab-org/gitlab-ce/issues/35186.

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!