I just read through this article https://www.mailpoet.com/post-notify-solutions-wordpress/
It is obviously not an exhaustive list, but unfortunately isn't WPMU's present.
Maybe the author is aware of this, maybe not. Either way, by looking at some of the other solutions, I think that you should consider improving the Subscribe by Email plugin in two ways:
1) Add stats to the service, so the user can see how many subscribers have opened the emails, and if possible, who has clicked on the "read more" link in the email.
2) Add a page-builder feature, that allow the user to create a more sleek design ( or create pre-made templates ).
This is a widely used feature in WordPress, so maybe it would be a smart move to do something about this specific request, as it could boost your reputation and plugin sale :smiley: