Touch ID and passcode protection for every app would protect our data no matter where it is.
With iOS 9.3 Apple will be giving us the ability to secure notes in the Notes app with TouchID or a password. It's a great way to keep personal information protected, but only if you keep it in Notes. Imagine if Apple took the feature and made it system-wide. That way, whether it's your photos or messages, your mail or health data, you'd be able to secure it with Touch ID or a password.
Touch ID everywhere
That's far, far easier to write than to engineer of course. Apps are interconnected. With Photos, there's the ImagePicker that lets you pull images and videos into other apps. With Messages, there's the Share Sheet extension that lets you send almost anything to almost any one. With Health, there's syncing data both into and out of other apps and even accessories.
Managing all of that at the app level — which is what the Touch ID API allows everything from password managers to bank apps to do already — is one thing. Managing all of that at the individual item level us quite another.
In other words, you don't get asked for Touch ID or a password simply to open the Notes app. You get asked for Touch ID or a password to open protected notes inside the app. That same kind of granularity could apply to other apps, depending on the type of data they contain.
For example, Messages could allow the protection of individual conversation threads. Health of entire categories. Photos could offer both protected Albums and protected pictures and videos.
Albums would require passcode or Touch ID to open, individual pictures and videos would be visible and accessible only when the protected album was unlocked. Neither would show up in Moments, Collections, Years, or ImagePickers outside the protected album.
Protecting the entire app, like a password manager or bank client, could make sense for Notes, Messages, Health, and other apps as well. It could be enabled on an app-by-app basis in Settings, the same way Notifications or Location are. Toggle on and you're asked for your passcode or Touch ID. (Whether or not per-app passcodes would be beneficial or burdensome is another question.)
App Store apps could even move their Touch ID options out of the per-app settings and into the unified Touch ID settings, making for a more consistent experience.
Protecting the individual items, like in Notes for iOS 9.3 might only make sense in some specific cases. Either way, just like Notes, it would let us keep any health, financial, picture, document, password, or other data safe and secure, regardless of what it is or where we keep it.
(If done right, it might even obviate the need for a Guest Mode and GuestBoard.)
Here's hoping Apple is already working on something like that for iOS 10... or beyond!
I've filed this as a feature request with Apple: rdar://24817355. If you have any ideas about securing iOS at the app or item level, let me know below!