Idea: Unlock-dependent modes.
It's really hard to describe how this app would work, so I'll do my best:
A lock screen that could unlock the phone with different passwords. For example, if you put in a password we'll call Password 'A', the phone would unlock normally. But say if you give your phone to your friend, you would put in Password 'B' which would unlock the phone, but simply put a lock on apps that you don't want them to go looking through. But what if you just have things on your phone like wallpapers or apps that you wouldn't show to a family member, then you would put in Password 'C' which would probably take some time to load because it would actually load up an entirely different home screen with a different wallpaper and would hide apps that you wouldn't want to be seen. Basically, one password would be meant for your personal use. The next password would be if you're just going to let your friend borrow your phone to look something up or use it just for a few minutes, and you wouldn't let them have access to certain things. The next password would be total-makeover mode, where the wallpaper and the home screen would be entirely changed.
This would probably only be able to work on higher-end devices with a lot of RAM available, and the size of the app would be very large. This is because the app would need to have a lock screen, set up and store preferences for each password unlock, as well as practically being a home screen replacement and being able to transition between modes.