Scale to zero - paid and preview apps
It would be super-useful for paid apps be able to sleep. It would save extra costs and improve loyalty to the platform.
Heroku has the same issue - review apps are not going to sleep at all and it is a great waste of money for idle services.
However, there are 3rd-party add-ons that enables the feature (on Heroku), but from Render I'd love to see this feature built-in.