A (simple?) proposal in the way to achieve {T3670}:
Add a Projects field in Projects, exactly the same that Maniphest tasks have. The main purpose being an aid to describe a project and a navigation cue to related projects.
This would open many more possibilities (semantic relationships, big overview of a Phabricator instance, etc) but just for the sake of getting things done, I would ignore all that in order to provide this simple and sweet feature sooner than later.
I have started doing this manually in project descriptions and I find it useful. Hoever, this is not as inviting and convenient as a dedicated field with type-ahead. See for instance https://phabricator.wikimedia.org/tag/phabricator/