Personalized content can quadruple your open rates. The key is sending content your users want to see based on actions they've taken - what content they've viewed and when they viewed it.
Anything you know about your users can be utilized to personalize. For example, if your users sign up with their name, include that in the message, so the notification comes off as a friendly note.
If you have a site or app where users can interact with each other, notifications are a great way to include the social activity taken by those users. These are most commonly used by marketplaces, social networks, and sites/apps that allow user comments.
Browser notifications are an extremely effective way to bring people back. We use them to tell our community the moment a notable product launches and when users are mentioned in comments.