Series? Pardon me? One multi-color status LED should suffice.
I mean… if they would do what everybody suggested, we would end up with…
A series of LEDs, a few unassigned extra buttons, a dozen of killswitches (for each sensor), the “classic” buttons, 3…6 cams (front, back, stereo, wide/zoom, infrared, …) + 2 flashes, finger print, a sliding hardware keyboard with a trackball, NFC, IR and rubber feet 
On a more serious note:
The existing buttons (+ and -) can in theory be repurposed. Like some camera apps do for zooming. Admittedly, this has some limits 
Another possibility would be to make real use of the gyroscope so you can shake and knock your phone and assign actions to that.