Short post, I've just had these ideas bouncing in my head for a while and it keeps annoying me phones have never implemented them.
First is a "guest lock". There's a better name for it somewhere but it's not my job to find it. Android and iOS should have a system-level ability to lock focus on the active application. This would mean you could open a concert ticket, a photo, or your digital insurance record (which is a VERY BAD IDEA to hand to a cop without this feature), guest-lock your phone, and hand it over without any worry that someone could start opening other apps to look for things you didn't want to show them.
The UX would be simple; inputting any command that would unfocus the current app would present a standard system unlock prompt. This means it would be more easily abused if you're using biometric unlocks, yes, but by no means does it make the feature useless. And it normalizes a level of trust between individuals which is appropriate, but impossible, in many situations.
A really nice feature would be a hardware button triggering this mode! Imagine how easy that would be to hand over your phone with vanishingly little worry that it could be abused.
You'll notice I mentioned showing a photo earlier. Obviously, the standard guest lock implementation doesn't prevent someone from scrolling your entire camera roll. A really good implementation would support specific locking features, such as setting a range of allowed photos, beyond which the guest user can't scroll. But that increases friction a lot, and leads to my second idea.
The second thing I would like is some form of hiding images from browsing in the camera roll. This could mean marking certain images as requiring a privileged viewing mode, or else they're completely hidden. Or it could mean defining "sets", which could be hidden as a group, or could hook into the above guest lock feature, or even other system features like mass-uploading. I bet there are other ways this could work, too.
If the need for this latter feature isn't obvious to you, well, that's ok. It would be very useful to me.