Here's a controversial position: This is not going to be popular amongst iOS devs (I am an iOS dev) but the best thing for iOS as a platform is for Apple to appropriate this.
With the M7 coprocessor, iOS would be able to register a callback for a given "gesture," like two knocks. Then the phone could implement such functionality while staying in standby! Combined with a facility for securely determining proximity, you could use such a facility to securely unlock everything.
How you reconcile which app gets which gesture then? Would two unlocks trigger both a machine unlock, a text to my mom, and to start playing my podcasts? Not to mention that it's like pulling teeth to get Apple to allow things to happen in the background.
I was intrigued at how they did this from a locked phone, and it turns out they don't. You have to have the app launched before knocking, meaning at it's core, this is just a shake listener with a count and some Bluetooth code. Not that it's not worth $4 (yeah, app isn't free), but it's not getting my $4 yet, since typing my password is still easier and plenty secure.
With the M7 coprocessor, iOS would be able to register a callback for a given "gesture," like two knocks. Then the phone could implement such functionality while staying in standby! Combined with a facility for securely determining proximity, you could use such a facility to securely unlock everything.