We have some Tasks on our Phabricator Instance which depend on stuff getting fixed upstream.
Usually we put a Link to the relevant upstream Bug/Ticket/whatever into our Task and from time to time check for changes and update the Task accordingly.
Maybe Maniphest could scan for external URLs it recognizes (for example Github) and actually include references, maybe even as a subtask.
The tasks could be detected by URL and recognized inside the Description, just like a Mention.
For private trackers an Admin could define Credentials which are automatically used, if they match a registred URL.
Probably GitHub, Bugzilla, Jira, Redmine or Trac are good candidates, at least when checking the usual open source projects for which Bugtracker they use.
I'm not sure if this would be the scope of T3179 since that handles actually importing Ticket Data, which isn't the scope of this Feature Request.