Implementing a notification system.
This notification system would function at the string level and would allow translators to open a text box linked to that specific string.

Who to notify.
Having the possibility to share this notification with every other language in the same project that contains the same source string.
Having the possibility of sending an automated notification to the author(s).

The reply.
Having the capacity to integrate the author’s reply (e.g. from an email) into the comments (e.g. into Zanata) in an automated manner would be very good.

Further notes/ideas:

The sending and receiving of notifications to and from authors should also probably be in the form of emails, to make sure the authors receive it.
As many people can add their own text to the comment box, it might be best if no-one else than the text’s author (or the project maintainer) can delete it.
Highlighting the strings that contain comments would be very useful.
Making the comments persistent across pushes would be very important.