We have currently already 3 Methods to unlock the device, right?
1.) Click on the power Button once and use your finger to slide the regulator right
2.) open the camera slot to go from locked to unlocked camera mode directly
3.) use the lock-button (switch it up to lock or to unlock)

I think i'll be only using method 3, because it's the fastest method to go back to your homescreen & I honestly think we don't need any more methods, because things like double click would require an enabled touchscreen in lockmode even when the screen doesn't show anything, which means more battery consumption instead of pressing a physical button.

edit: When it comes to security, touchscreens can't scan fingerprints unfortunately, so I think we should stick to a predefined gesture that unlocks the device. Voice unlocking should be possible, too, but I wouldn't prefer that, since you need to unlock your phone in the public aswell.