The ability to let customers signup for a Membership/Non-booking subscription, with customers being able to manage/cancel this subscription from their profile.
Here are a couple use cases that would be amazing for my Seasonal mobile car detailing business:
Customers subscribe to a $99/year membership plan.
When The Customer Is Billed, They Receive $X (example: $99 + $20 BONUS) In Credits (or X number of appointments) they can Apply To Any Future Booking.
Admins can set the membership type to be a fixed billing date (ex. April 1 of each year), or let the billing date be immediately upon signup.