Building on the subscription data model already in place, create ability to subscribe (and receive notifications) on content items.
This will need to integrate with the scheduler module in order to schedule the sending of these notifications (since content can be future-published).
Add flags for whether content has been notified already. This will help prevent dupe notifications, especially after migrating content
Add ability to subscribe to hierarchies of content, as well as content in general. For example, if someone subscribes to the blog, they should receive all notifications for new blog entries. If someone subscribes to a particular page, they should receive notifications for new children of those pages.
Add a way to designate per content whether it's notifiable or not