The Due app does something like this. Basically it’s 1 time fee + subscription. The day you buy the app, you get every new feature for the next year and every previous feature. If a new feature is added, you can subscribe for $5 a year. Upon subscribing, you get all new features since your subscription lapsed. Blog post here: https://www.dueapp.com/blog/future-of-paid-upgrades.html