Any update on this? I've seen this feature in other forum software, and experienced it first hand on a number of communities I have registered on and it is really quite useful to receive a weekly digest (and preferably a custom schedule, whether it be daily, weekly, monthly depending on how active your forum) to get an overview of popular topics.
It could also be useful as an administrative function to get highlights for your community for your own awareness. To see what the major issues are, or what is trending that you can then use to make business decisions on. So if there were configurations, to create multiple instances of a digest, and in each you could configure the schedule, target users based on certain criteria (e.g. specific groups, all users, active within last X days), tag filter, etc. I think that would be absolutely amazing. I wish you luck in the development, I know it will be a hit here and one of the more popular extensions.