I've got a working MVP that I want to test with some real users to get feedback on whether this is useful and something worth continuing to pursue. See a live demo here: http://example.ravenhub.io/
The idea is that you can add an in-app notification center to your web app with a line of html and a javascript tag. Then send custom templated notifications to your users with a REST API.
In theory you should be able to add this to your app pretty easily so you can focus on core functionality. It also includes activity tracking, a Slack integration and templates with emojis of course!
Please contact me if you're interested (or just sign up on the site). You can take a look at the docs here to get an idea of the effort it would take to add to your app (link to full documentation in the link above)