Killer feature for email: add an email header that turns an email to notification. Then email apps would push the notification to the user, bypassing apple’s block. Websites already have the user’s email, so no need for more subscriptions. If gmail implemented this, it might push for adoption
Tangentially related, iOS mail app has a VIP setting.
I've seen poor push notification quality increase uninstall and churn rates. I think the lesson is that many people are quickly annoyed with push notifications.
Let's not assume that all push notifications are cynical attempts to get people engaged with the app or to push DLC. For things like mail, messaging, and plenty of other online services, push notifications are not just an important interface to the app; they can be the interface.
Notifications have fine tuned controls on both iOS and Android with schedule Do Not Disturb features and varying levels of priority (iOS has 3 tiers). Plus per-app controls to disable them from annoying apps, which in newer versions of iOS prompt you before they turn on, making them opt-in.
It's essential for communication apps (texting, email) and useful for breaking news or a simple highlight of big news on WSJ/NYT on your activity feed, so you don't need to open either app to stay on top of big events if you're a news junkie like me.
I'd personally go as far as saying push-notifications + the activity feed is very underrated feature of smart phones and has become a core part of people's day-to-day lives (which plenty of technologies can't say).