Happy to see Newsletters being discussed! [1]
What matters is to allow users to receive a notification via web or email (according to their preferences) pointing to a newly posted edition.
Let's forget about Flow for now: there are many problems to solve before addressing discussions in newsletters.
I think we can also forget about posting full newsletters in Talk pages. Bots are already doing this, if anybody is interested.
After some thoughts and the priceless feedback and help from Siebrand (who runs the TranslateWiki.net newsletter), we decided to start with
https://www.mediawiki.org/wiki/Extension:Newsletter
Today it simply contains a user preference allowing autoconfirmed email users to opt in to one newsletter. The email from these users can be retrieved from the database for a manual mailing.
This is VERY pedestrian, but it's a start. See some draft specs at
https://www.mediawiki.org/wiki/Extension_talk:Newsletter#Drafting_specs_3325...
I guess the right step is to make an Echo extension? Your guidance and help is welcome.
[1] See Engaging users with the old plain newsletter http://lists.wikimedia.org/pipermail/ee/2013-August/000616.html