Replies to notification emails become comment on the post
Being able to respond to post subscription by replying to the email notification itself. (Like https://wordpress.org/plugins/postmatic/ )
I understand this is a bigger undertaking. I would pay for this plugin separately, so long as it worked with or was an add-on to the Follow My Blog Post plugin. The plugin in the example appears to no longer be supported, so you could bring in users of that, everyone looking for that and upgrade current users.