IMHO this will be nice to have several pushbuttons on one or more side (in side_frame, not on front and not on rear) of phone with user assignable functions.
User can assign such functions as play, pause/stop, next or previous track, accept call, reject call, etc…
This allow to activate required function even without unlocking screen.
The pushbutton(s) must not to protrude from side_frame flatness to eliminate unwanted activation when holding and transportation.
IMHO flatness of side_frame and flatness of pushbutton (when not pressed) must be the same (equal, to form single surface).
And such pushbuttons must have large enough diameter (more than 6 mm) for convenient manipulating (pushing) by human fingers.
Of course dedicated lock slider switch necessary to logically lock this pushbuttons from acting when user in doubt about unwanted occasional push.
(e.g. cusped objects in the bag with phone)
It may be groundbreaking to inflict braille character (or some like) on pushbutton top to simplify blind using.
It’s not certain to me how many and which side pushbuttons should be. So invite to offer your opinions.
Below the scheme to simplify formulate and exclude uncertainity.
Side naming. Phone view from front (screen).
N (North)
________
| |
| |
| |
W (West) | front | E (East)
| |
| screen |
| |
| |
|________|
S (South)
Expression elements.
Expression similar to equation. Left part correspond to side naming and right part correspond to quantity.
There is combinatorial operations.
“+” “summing” sides without order accounting (W+E=8 mean exactly 8 pushbuttons in sum for W and E sides (i.e. both long sides)
“^” exclusively one side (W^E=5 mean exactly 5 pushbuttons on one of long (W or E) side only)
“|” exchange sides (N|W=5|3 mean 5 pushbuttons on one long side and 3 pushbuttons on opposite long side)
“?” before quantity mean “I’m not sure about exact quantity” (more “?” more unsure) (N=?1 mean 1 pushbutton on N side, but I’m not sure
“+” after quantity mean not less than (W=5+ mean not less than 5 pushbuttons on W side)
“-” after quantity mean not more than (E=3- mean not more than 3 pushbuttons on E side)
“~” before quantity mean about (W+E=~8 mean about 8 pushbuttons in sum for long sides)
“…” between numbers mean quantity interval (N+S+W+E=~8…13 mean overall pushbuttons quantity is about from 8 to 13)
Probably not all cases can be expressed via this expressions, so if not so lazy, write expanded opinion 
IMHO nice to have several user assignable RGB LEDs on front side (probably above screen area) or maybe inside side frame emiting light to side.
User can assign this LEDs to indicating various kinds of events: SMS arrival, missed call, e-mail arrival, new message, loss of network, alarms, etc.
This allow to control main and necessary events even with main screen fully off, and save battery energy.
Use same equation for LEDs with addition of F (Front) and B (Back,Rear) sides.
IMHO i.MX8M GPIO1.IO[0]…GPIO1.IO[15] is preferred for pushbuttons connections because it has no multiplexing with other functions, hence it will not interfere with any specific peripheral in present and future designed devices.
And for pushbuttons GPIO1.IO[0]…GPIO1.IO[7] better to use (unless used for IRQ signal from external peripheral module), because it has dedicated connection to interrupt controller.
For RGB LEDs seems better to use specialized control driver ICs which already has current driver and digital color (RGB component) input via SPI or I2C.
Resume:
My opinion is W|E=5+|5- I.e. Five or more pushbuttons on one long side plus not more than five pushbuttons on opposite side.
And with no sentence about short sides, but most probably 0 pushbuttons there.
For LED: F=~3…5 I.e. about 3 to 5 LEDs on front.
