It already does work like this in iOS 11. Apps can present the System photo picker to you and receive only your selected photo while having their Photos access set to "Never".
If you want to try it out install the Wire messenger (if you make the account with a web browser you don't need to provide a phone number), and try to attach a photo but deny Photo library permissions. (Here's the buttons to press: https://imgur.com/a/gc5Iq). Other apps work this way on iOS 11 but this is the one that came to mind.
If you want to try it out install the Wire messenger (if you make the account with a web browser you don't need to provide a phone number), and try to attach a photo but deny Photo library permissions. (Here's the buttons to press: https://imgur.com/a/gc5Iq). Other apps work this way on iOS 11 but this is the one that came to mind.